#![allow(unused_macros, deprecated)]

#[allow(dead_code)]
mod utils {
    use std::collections::{HashMap, HashSet};
    use std::hash::{BuildHasherDefault, Hash, Hasher};
    use std::io::{
        self, BufRead, BufReader, BufWriter, Seek, SeekFrom, Stdin, StdinLock, Stdout, StdoutLock,
        Write,
    };
    use std::mem;
    use std::ops::{Deref, DerefMut};
    use std::ptr;
    use std::slice;
    use std::str::{self, FromStr};

    #[inline]
    fn is_space_or_line_break(b: &u8) -> bool {
        *b == b' ' || *b == b'\n'
    }

    pub struct Scanner<R: BufRead> {
        reader: R,
        buf: Vec<u8>,
    }

    impl<R: BufRead> Scanner<R> {
        #[inline]
        pub fn new(reader: R) -> Scanner<R> {
            Scanner {
                reader: reader,
                buf: Vec::with_capacity(100),
            }
        }

        pub fn read<T: FromStr>(&mut self) -> T {
            self.read_until(is_space_or_line_break);
            self.parse()
        }

        pub fn read2<T1: FromStr, T2: FromStr>(&mut self) -> (T1, T2) {
            (self.read(), self.read())
        }

        pub fn read3<T1: FromStr, T2: FromStr, T3: FromStr>(&mut self) -> (T1, T2, T3) {
            (self.read(), self.read(), self.read())
        }

        pub fn read4<T1: FromStr, T2: FromStr, T3: FromStr, T4: FromStr>(
            &mut self,
        ) -> (T1, T2, T3, T4) {
            (self.read(), self.read(), self.read(), self.read())
        }

        pub fn reads(&mut self) -> &str {
            self.read_until(is_space_or_line_break);
            unsafe { str::from_utf8_unchecked(&self.buf) }
        }

        pub fn readl(&mut self) -> &str {
            self.buf.clear();
            self.reader.read_until(b'\n', &mut self.buf).unwrap();
            let bytes = if self.buf.last() == Some(&b'\n') {
                &self.buf[..self.buf.len() - 1]
            } else {
                &self.buf
            };
            unsafe { str::from_utf8_unchecked(&bytes) }
        }

        pub fn read_vec<T: FromStr>(&mut self, count: usize) -> Vec<T> {
            (0..count).map(|_| self.read()).collect()
        }

        fn read_until<P: FnMut(&u8) -> bool>(&mut self, mut predicate: P) {
            self.buf.clear();
            let mut first = true;

            // https://doc.rust-lang.org/src/std/io/mod.rs.html#1623-1649
            loop {
                let (done, used) = {
                    let available = self.reader.fill_buf().unwrap();
                    if first && available.len() == 0 {
                        panic!("Scan Error (EOF reached)");
                    }

                    match available.iter().position(&mut predicate) {
                        Some(0) if first => (false, 1),
                        Some(i) => {
                            self.buf.extend_from_slice(&available[..i]);
                            (true, i + 1)
                        }
                        None => {
                            self.buf.extend_from_slice(available);
                            (false, available.len())
                        }
                    }
                };

                self.reader.consume(used);
                if done || used == 0 {
                    return;
                }

                first = false;
            }
        }

        #[inline]
        fn parse<T: FromStr>(&mut self) -> T {
            // 文字列をT型へパースする
            let elem = unsafe { str::from_utf8_unchecked(&self.buf) };
            elem.parse()
                .unwrap_or_else(|_| panic!(format!("failed parsing: {}", elem)))
        }
    }

    impl<R: BufRead + Seek> Seek for Scanner<R> {
        #[inline]
        fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> {
            self.reader.seek(pos)
        }
    }

    pub struct StdinScanner {
        stdin: Stdin,
        scanner: Scanner<BufReader<StdinLock<'static>>>,
    }

    impl StdinScanner {
        #[inline]
        pub fn new() -> StdinScanner {
            let stdin = io::stdin();
            let stdinlock = unsafe { mem::transmute(stdin.lock()) };
            let reader = BufReader::with_capacity(4096, stdinlock);
            let scanner = Scanner::new(reader);
            StdinScanner {
                stdin: stdin,
                scanner: scanner,
            }
        }
    }

    impl Deref for StdinScanner {
        type Target = Scanner<BufReader<StdinLock<'static>>>;

        #[inline]
        fn deref(&self) -> &Self::Target {
            &self.scanner
        }
    }

    impl DerefMut for StdinScanner {
        #[inline]
        fn deref_mut(&mut self) -> &mut Self::Target {
            &mut self.scanner
        }
    }

    pub struct StdoutWriter {
        stdout: Stdout,
        writer: BufWriter<StdoutLock<'static>>,
    }

    impl StdoutWriter {
        #[inline]
        pub fn new() -> StdoutWriter {
            let stdout = io::stdout();
            let stdoutlock = unsafe { mem::transmute(stdout.lock()) };
            let writer = BufWriter::with_capacity(4096, stdoutlock);
            StdoutWriter {
                stdout: stdout,
                writer: writer,
            }
        }

        pub fn write<T: Writable>(&mut self, v: T) {
            v.display(&mut self.writer);
        }

        pub fn writeln<T: Writable>(&mut self, v: T) {
            v.display(&mut self.writer);
            self.writer.write_all(b"\n").unwrap();
        }

        pub fn write_horizontal<I>(&mut self, iter: I)
        where
            I: IntoIterator,
            I::Item: Writable,
        {
            let mut it = iter.into_iter();
            if let Some(elem) = it.next() {
                elem.display(&mut self.writer);
            } else {
                return;
            }

            for elem in it {
                elem.display(&mut self.writer);
                self.writer.write_all(b" ").unwrap();
            }
        }

        pub fn write_vertical<I>(&mut self, iter: I)
        where
            I: IntoIterator,
            I::Item: Writable,
        {
            let mut it = iter.into_iter();
            if let Some(elem) = it.next() {
                elem.display(&mut self.writer);
            } else {
                return;
            }

            for elem in it {
                elem.display(&mut self.writer);
                self.writer.write_all(b"\n").unwrap();
            }
        }
    }

    impl Write for StdoutWriter {
        #[inline]
        fn flush(&mut self) -> io::Result<()> {
            self.writer.flush()
        }

        #[inline]
        fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
            self.writer.write(buf)
        }
    }

    #[derive(Clone)]
    pub struct FNVHasher(u64);

    impl Default for FNVHasher {
        fn default() -> FNVHasher {
            FNVHasher(0xcbf29ce484222325)
        }
    }

    impl Hasher for FNVHasher {
        #[inline]
        fn write(&mut self, bytes: &[u8]) {
            let FNVHasher(mut hash) = *self;

            for byte in bytes.iter() {
                hash = hash ^ (*byte as u64);
                hash = hash.wrapping_mul(0x100000001b3);
            }

            *self = FNVHasher(hash);
        }

        #[inline]
        fn finish(&self) -> u64 {
            self.0
        }
    }

    pub type FNVSet<T> = HashSet<T, BuildHasherDefault<FNVHasher>>;
    pub type FNVMap<K, V> = HashMap<K, V, BuildHasherDefault<FNVHasher>>;

    #[inline]
    pub fn fnvset_new<T: Eq + Hash>() -> FNVSet<T> {
        FNVSet::<T>::with_hasher(BuildHasherDefault::default())
    }

    #[inline]
    pub fn fnvset_with_capacity<T: Eq + Hash>(n: usize) -> FNVSet<T> {
        FNVSet::<T>::with_capacity_and_hasher(n, BuildHasherDefault::default())
    }

    #[inline]
    pub fn fnvmap_new<K: Eq + Hash, V>() -> FNVMap<K, V> {
        FNVMap::<K, V>::with_hasher(BuildHasherDefault::default())
    }

    #[inline]
    pub fn fnvmap_with_capacity<K: Eq + Hash, V>(n: usize) -> FNVMap<K, V> {
        FNVMap::<K, V>::with_capacity_and_hasher(n, BuildHasherDefault::default())
    }

    static DEC_DIGITS_LUT: &'static [u8; 200] = b"0001020304050607080910111213141516171819\
            2021222324252627282930313233343536373839\
            4041424344454647484950515253545556575859\
            6061626364656667686970717273747576777879\
            8081828384858687888990919293949596979899";

    fn write_int<W: Write>(mut writer: W, mut n: u64, is_negative: bool) {
        unsafe {
            let mut buf: [u8; 39] = mem::uninitialized();
            let buf_ptr_start = buf.as_mut_ptr().offset(39);
            let mut buf_ptr = buf_ptr_start;
            let lut_ptr = DEC_DIGITS_LUT.as_ptr();

            while n >= 10000 {
                let rem = (n % 10000) as isize;
                n /= 10000;

                let d1 = (rem / 100) << 1;
                let d2 = (rem % 100) << 1;
                buf_ptr = buf_ptr.offset(-4);
                ptr::copy_nonoverlapping(lut_ptr.offset(d1), buf_ptr, 2);
                ptr::copy_nonoverlapping(lut_ptr.offset(d2), buf_ptr.offset(2), 2);
            }

            let mut n = n as isize; // possibly reduce 64bit math

            // decode 2 more chars, if > 2 chars
            if n >= 100 {
                let d1 = (n % 100) << 1;
                n /= 100;
                buf_ptr = buf_ptr.offset(-2);
                ptr::copy_nonoverlapping(lut_ptr.offset(d1), buf_ptr, 2);
            }

            // decode last 1 or 2 chars
            if n < 10 {
                buf_ptr = buf_ptr.offset(-1);
                *buf_ptr = (n as u8) + b'0';
            } else {
                let d1 = n << 1;
                buf_ptr = buf_ptr.offset(-2);
                ptr::copy_nonoverlapping(lut_ptr.offset(d1), buf_ptr, 2);
            }

            if is_negative {
                buf_ptr = buf_ptr.offset(-1);
                *buf_ptr = b'-';
            }

            let buf_slice =
                slice::from_raw_parts(buf_ptr, buf_ptr_start as usize - buf_ptr as usize);

            writer.write_all(buf_slice).unwrap();
        }
    }

    pub trait Writable {
        fn display<W: Write>(&self, writer: &mut W);
    }

    impl Writable for str {
        #[inline]
        fn display<W: Write>(&self, writer: &mut W) {
            writer.write_all(self.as_bytes()).unwrap()
        }
    }

    impl<'a> Writable for &'a str {
        #[inline]
        fn display<W: Write>(&self, writer: &mut W) {
            writer.write_all(self.as_bytes()).unwrap()
        }
    }

    impl Writable for String {
        #[inline]
        fn display<W: Write>(&self, writer: &mut W) {
            writer.write_all(self.as_bytes()).unwrap()
        }
    }

    impl Writable for bool {
        #[inline]
        fn display<W: Write>(&self, writer: &mut W) {
            let s: &'static [u8] = if *self { b"Yes" } else { b"No" };
            writer.write_all(s).unwrap();
        }
    }

    macro_rules! impl_writable_int {
        ($($int:ty)*) => {
            $(
                impl Writable for $int {
                    #[allow(unused_comparisons)]
                    fn display<W: Write>(&self, writer: &mut W) {
                        let is_negative = !(*self >= 0);
                        let abs = if is_negative {
                            (!*self as u64).wrapping_add(1)
                        } else {
                            *self as u64
                        };
                        write_int(writer, abs, is_negative);
                    }
                }
            )*
        };
    }

    macro_rules! impl_writable_float {
        ($($float:ty)*) => {
            $(
                impl Writable for $float {
                    #[inline]
                    fn display<W: Write>(&self, writer: &mut W) {
                        write!(writer, "{}", *self).unwrap();
                    }
                }
            )*
        }
    }

    impl_writable_int!(u8 u16 u32 u64 usize i8 i16 i32 i64 isize);
    impl_writable_float!(f32 f64);
}

fn main() {
    let mut scanner = utils::StdinScanner::new();
    let mut writer = utils::StdoutWriter::new();

    let (n, k) = scanner.read2::<usize, usize>();
    let p = scanner.read_vec::<u64>(n);

    let mut sum: u64 = p[0..k].iter().sum();
    let mut max_sum = sum;

    for (a, b) in p.iter().zip(p[k..].iter()) {
        sum += *b;
        sum -= *a;
        if sum > max_sum {
            max_sum = sum;
        }
    }

    writer.writeln((k as u64 + max_sum) as f64 / 2.0);
}


// The main code is at the very bottom.

#[allow(unused_imports)]
#[macro_use]
pub mod spella {
  pub mod byte {
    pub use self::byte_char::*;
    pub use self::byte_str::*;
    pub use self::byte_string::*;
    pub use self::from_byte_str::*;

    mod byte_char {
      use std::fmt::{self, Debug, Display, Formatter};

      #[derive(Clone, Copy, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
      pub struct ByteChar(pub u8);

      impl Debug for ByteChar {
        fn fmt(&self, f: &mut Formatter) -> fmt::Result {
          write!(f, "b'{}'", self.0 as char)
        }
      }

      impl Display for ByteChar {
        fn fmt(&self, f: &mut Formatter) -> fmt::Result {
          write!(f, "{}", self.0 as char)
        }
      }
    }

    mod byte_str {
      use spella::byte::{ByteChar, ByteString};
      use std::fmt::{self, Debug, Display, Formatter};
      use std::ops::{Deref, DerefMut};

      #[derive(PartialEq, Eq, PartialOrd, Ord, Hash)]
      pub struct ByteStr([ByteChar]);

      macro_rules! cast {
        (mut $x:expr, $($T:ty)=>*) => {
          unsafe { &mut *($x $(as *mut $T)*) }
        };
        ($x:expr, $($T:ty)=>*) => {
          unsafe { &*($x $(as *const $T)*) }
        };
      }

      impl ByteStr {
        pub fn from_bytes(s: &[u8]) -> &Self {
          cast!(s, [u8] => [ByteChar] => ByteStr)
        }

        pub fn from_bytes_mut(s: &mut [u8]) -> &mut Self {
          cast!(mut s, [u8] => [ByteChar] => ByteStr)
        }

        pub fn from_byte_chars(s: &[ByteChar]) -> &Self {
          cast!(s, [ByteChar] => ByteStr)
        }

        pub fn from_byte_chars_mut(s: &mut [ByteChar]) -> &mut Self {
          cast!(mut s, [ByteChar] => ByteStr)
        }

        pub fn as_byte_chars(&self) -> &[ByteChar] {
          &self.0
        }

        pub fn as_byte_chars_mut(&mut self) -> &mut [ByteChar] {
          &mut self.0
        }

        pub fn as_bytes(&self) -> &[u8] {
          cast!(self, ByteStr => [ByteChar] => [u8])
        }

        pub fn as_bytes_mut(&mut self) -> &mut [u8] {
          cast!(mut self, ByteStr => [ByteChar] => [u8])
        }
      }

      impl ToOwned for ByteStr {
        type Owned = ByteString;

        fn to_owned(&self) -> ByteString {
          ByteString::from(self.0.to_owned())
        }
      }

      impl Debug for ByteStr {
        fn fmt(&self, f: &mut Formatter) -> fmt::Result {
          write!(f, "b\"")?;

          for &c in &self.0 {
            write!(f, "{}", c)?;
          }

          write!(f, "\"")
        }
      }

      impl Display for ByteStr {
        fn fmt(&self, f: &mut Formatter) -> fmt::Result {
          for &c in &self.0 {
            write!(f, "{}", c)?;
          }

          Ok(())
        }
      }

      impl Deref for ByteStr {
        type Target = [ByteChar];

        fn deref(&self) -> &[ByteChar] {
          self.as_byte_chars()
        }
      }

      impl DerefMut for ByteStr {
        fn deref_mut(&mut self) -> &mut [ByteChar] {
          self.as_byte_chars_mut()
        }
      }
    }

    mod byte_string {
      use spella::byte::{ByteChar, ByteStr};

      use std::borrow::{Borrow, BorrowMut};
      use std::fmt::{self, Debug, Display, Formatter};
      use std::ops::{Deref, DerefMut};

      #[derive(Clone, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
      pub struct ByteString(Vec<ByteChar>);

      impl ByteString {
        pub fn into_byte_chars(self) -> Vec<ByteChar> {
          self.0
        }

        pub fn as_byte_str(&self) -> &ByteStr {
          ByteStr::from_byte_chars(&self.0)
        }

        pub fn as_mut_byte_str(&mut self) -> &mut ByteStr {
          ByteStr::from_byte_chars_mut(&mut self.0)
        }
      }

      impl From<Vec<ByteChar>> for ByteString {
        fn from(s: Vec<ByteChar>) -> ByteString {
          ByteString(s)
        }
      }

      impl Borrow<ByteStr> for ByteString {
        fn borrow(&self) -> &ByteStr {
          self.as_byte_str()
        }
      }

      impl BorrowMut<ByteStr> for ByteString {
        fn borrow_mut(&mut self) -> &mut ByteStr {
          self.as_mut_byte_str()
        }
      }

      impl Debug for ByteString {
        fn fmt(&self, f: &mut Formatter) -> fmt::Result {
          Debug::fmt(self.as_byte_str(), f)
        }
      }

      impl Display for ByteString {
        fn fmt(&self, f: &mut Formatter) -> fmt::Result {
          Display::fmt(self.as_byte_str(), f)
        }
      }

      impl Deref for ByteString {
        type Target = ByteStr;

        fn deref(&self) -> &ByteStr {
          ByteStr::from_byte_chars(&self.0)
        }
      }

      impl DerefMut for ByteString {
        fn deref_mut(&mut self) -> &mut ByteStr {
          ByteStr::from_byte_chars_mut(&mut self.0)
        }
      }
    }

    mod from_byte_str {
      use spella::byte::{ByteChar, ByteStr, ByteString};

      use std::error::Error;
      use std::fmt::{self, Debug, Display, Formatter};
      use std::str::{self, FromStr, Utf8Error};

      pub trait FromByteStr: Sized {
        type Err;

        fn from_byte_str(s: &ByteStr) -> Result<Self, Self::Err>;
      }

      macro_rules! fn_description {
        () => {
          fn description(&self) -> &str {
            "description() is deprecated; use Display"
          }
        };
      }

      #[derive(Debug)]
      pub struct ParseByteCharError(ParseByteCharErrorKind);

      #[derive(Debug)]
      enum ParseByteCharErrorKind {
        EmptyByteStr,
        TooManyByteChars,
      }

      impl Display for ParseByteCharError {
        fn fmt(&self, f: &mut Formatter) -> fmt::Result {
          use self::ParseByteCharErrorKind::*;

          f.write_str(match self.0 {
            EmptyByteStr => "empty `ByteStr`",
            TooManyByteChars => "too many `ByteChar`s",
          })
        }
      }

      impl Error for ParseByteCharError {
        fn_description! {}
      }

      impl FromByteStr for ByteChar {
        type Err = ParseByteCharError;

        fn from_byte_str(s: &ByteStr) -> Result<Self, Self::Err> {
          use self::ParseByteCharErrorKind::*;

          match s.len() {
            1 => Ok(unsafe { *s.get_unchecked(0) }),
            0 => Err(ParseByteCharError(EmptyByteStr)),
            _ => Err(ParseByteCharError(TooManyByteChars)),
          }
        }
      }

      #[derive(Debug)]
      pub enum ParseByteStringError {}

      impl Display for ParseByteStringError {
        fn fmt(&self, _: &mut Formatter) -> fmt::Result {
          match *self {}
        }
      }

      impl Error for ParseByteStringError {
        fn_description! {}
      }

      impl FromByteStr for ByteString {
        type Err = ParseByteStringError;

        fn from_byte_str(s: &ByteStr) -> Result<Self, Self::Err> {
          Ok(ByteString::from(s.to_vec()))
        }
      }

      pub struct ParseFromStrError<T: FromStr>(ParseFromStrErrorKind<T>);

      enum ParseFromStrErrorKind<T: FromStr> {
        Utf8Error(Utf8Error),
        FromStrError(T::Err),
      }

      impl<T: FromStr> Debug for ParseFromStrError<T>
      where
        T::Err: Debug,
      {
        fn fmt(&self, f: &mut Formatter) -> fmt::Result {
          use self::ParseFromStrErrorKind::*;

          match self.0 {
            Utf8Error(ref err) => f.debug_tuple("Utf8Error").field(err).finish(),
            FromStrError(ref err) => f.debug_tuple("FromStrError").field(err).finish(),
          }
        }
      }

      impl<T: FromStr> Display for ParseFromStrError<T>
      where
        T::Err: Display,
      {
        fn fmt(&self, f: &mut Formatter) -> fmt::Result {
          use self::ParseFromStrErrorKind::*;

          match self.0 {
            Utf8Error(ref err) => write!(f, "{}", err),
            FromStrError(ref err) => write!(f, "{}", err),
          }
        }
      }

      impl<T: FromStr> Error for ParseFromStrError<T>
      where
        T::Err: Debug + Display,
      {
        fn_description! {}
      }

      impl<T: FromStr> FromByteStr for T {
        type Err = ParseFromStrError<T>;

        fn from_byte_str(s: &ByteStr) -> Result<T, Self::Err> {
          use self::ParseFromStrErrorKind::*;

          str::from_utf8(s.as_bytes())
            .map_err(|e| ParseFromStrError(Utf8Error(e)))
            .and_then(|s| s.parse().map_err(|e| ParseFromStrError(FromStrError(e))))
        }
      }
    }
  }

  pub mod io {
    pub use self::scanner::*;

    mod scanner {
      use spella::byte::{ByteStr, FromByteStr};

      use std::io::{self, BufRead};

      #[derive(Debug)]
      pub struct Scanner<R> {
        reader: R,
        buf: Vec<u8>,
        pos: usize,
      }

      const INITIAL_CAPACITY: usize = 32;

      impl<R: BufRead> Scanner<R> {
        pub fn new(reader: R) -> Self {
          Scanner {
            reader: reader,
            buf: Vec::with_capacity(INITIAL_CAPACITY),
            pos: 0,
          }
        }

        pub fn next<T: FromByteStr>(&mut self) -> io::Result<Result<T, T::Err>> {
          self.next_byte_str().map(T::from_byte_str)
        }

        pub fn next_byte_str(&mut self) -> io::Result<&ByteStr> {
          if self.buf.is_empty() {
            self.read_line()?;
          }

          loop {
            match self.buf.get(self.pos) {
              Some(&b' ') => self.pos += 1,
              Some(&b'\n') => self.read_line()?,
              Some(_) => break,
              None => return Err(io::Error::from(io::ErrorKind::UnexpectedEof)),
            }
          }

          let start = self.pos;
          self.pos += 1;

          loop {
            match self.buf.get(self.pos) {
              Some(&b' ') | Some(&b'\n') | None => break,
              Some(_) => self.pos += 1,
            }
          }

          Ok(ByteStr::from_bytes(&self.buf[start..self.pos]))
        }

        fn read_line(&mut self) -> io::Result<()> {
          self.buf.clear();
          self.pos = 0;
          self.reader.read_until(b'\n', &mut self.buf)?;

          Ok(())
        }
      }
    }
  }
}

mod prelude {
  pub use spella::byte::{ByteChar, ByteStr, ByteString};
  pub use std::collections::*;
  pub use std::io::prelude::*;
  pub use std::iter::FromIterator;
  pub use std::marker::PhantomData;
  pub use std::num::Wrapping;
  pub use std::ops::{Range, RangeFrom, RangeTo};
  pub use std::{cell, cmp, f64, i32, i64, isize, iter, mem, rc, str, time, u32, u64, usize};
}

use prelude::*;

const CUSTOM_STACK_SIZE_MEBIBYTES: Option<usize> = None;

fn main() {
  fn exec_solver() {
    let stdin = std::io::stdin();
    let stdout = std::io::stdout();
    #[cfg(not(debug_assertions))]
    let mut writer = std::io::BufWriter::new(stdout.lock());
    #[cfg(debug_assertions)]
    let mut writer = stdout.lock();
    solve(stdin.lock(), &mut writer);
    writer.flush().unwrap();
  }

  if let Some(stack_size_mebibytes) = CUSTOM_STACK_SIZE_MEBIBYTES {
    std::thread::Builder::new()
      .name("exec_solver".to_owned())
      .stack_size(stack_size_mebibytes * 1024 * 1024)
      .spawn(exec_solver)
      .unwrap()
      .join()
      .unwrap();
  } else {
    exec_solver();
  }
}

fn solve<R: BufRead, W: Write>(reader: R, mut writer: W) {
  let mut _scanner = spella::io::Scanner::new(reader);

  #[allow(unused_macros)]
  macro_rules! scan {
    ($T:ty) => {
      _scanner.next::<$T>().unwrap().unwrap()
    };
    ($($T:ty),+) => {
      ($(scan!($T)),+)
    };
    ($($T:ty),+; $n:expr $(; $m:expr)*) => {{
      (0..$n).map(|_| scan!($($T),+ $(; $m)*)).collect::<Vec<_>>()
    }};
  }

  #[allow(unused_macros)]
  macro_rules! scan_iter {
    ($($T:ty),+; $n:expr) => {
      (0..$n).map(|_| scan!($($T),+))
    };
  }

  #[allow(unused_macros)]
  macro_rules! print {
    ($fmt:expr) => {
      write!(writer, $fmt).unwrap()
    };
    ($fmt:expr, $($arg:tt)*) => {
      write!(writer, $fmt, $($arg)*).unwrap()
    };
  }

  #[allow(unused_macros)]
  macro_rules! println {
    ($fmt:expr) => {
      writeln!(writer, $fmt).unwrap()
    };
    ($fmt:expr, $($arg:tt)*) => {
      writeln!(writer, $fmt, $($arg)*).unwrap()
    };
  }

  #[allow(unused_macros)]
  macro_rules! eprint {
    ($fmt:expr) => {
      #[cfg(debug_assertions)]
      write!(std::io::stderr(), $fmt).unwrap()
    };
    ($fmt:expr, $($arg:tt)*) => {
      #[cfg(debug_assertions)]
      write!(std::io::stderr(), $fmt, $($arg)*).unwrap()
    };
  }

  #[allow(unused_macros)]
  macro_rules! eprintln {
    ($fmt:expr) => {
      #[cfg(debug_assertions)]
      writeln!(std::io::stderr(), $fmt).unwrap()
    };
    ($fmt:expr, $($arg:tt)*) => {
      #[cfg(debug_assertions)]
      writeln!(std::io::stderr(), $fmt, $($arg)*).unwrap()
    };
  }

  #[allow(unused_macros)]
  macro_rules! dbg {
    ($($x:expr),+) => {{
      eprintln!(concat!("[{}:{}] ", $(stringify!($x), " = {:?}; "),+), file!(), line!(), $($x),+);
    }};
  }

  let (n, k) = scan!(usize, usize);
  let p = scan!(u64; n);

  let mut sum = p.iter().take(k).sum::<u64>();
  let mut max_sum = sum;
  let mut max_sum_range = 0..k;
  for i in 1..n - k + 1 {
    dbg!(i..i + k);
    sum -= p[i - 1];
    sum += p[i + k - 1];
    if sum > max_sum {
      max_sum_range = i..i + k;
      max_sum = sum;
    }
  }

  let ans = p[max_sum_range]
    .iter()
    .map(|&p_i| (p_i + 1) as f64 / 2.0)
    .sum::<f64>();
  println!("{}", ans);
}


use whiteread::parse_line;

fn main() {
    let (n, k): (usize, usize) = parse_line().unwrap();
    let s: Vec<u32> = parse_line().unwrap();
    let mut a: Vec<f32> = vec![0.0];
    let mut ans: f32 = 0.0;

    for i in 0..n {
        a.push(a[i] + (1 + s[i]) as f32 / 2.0);
    }

    for i in 0..n - k + 1 {
        ans = ans.max(a[i + k] - a[i]);
    }
    println!("{}", ans);
}