pub trait IterScan: Sized {
    type Output;
    fn scan<'a, I: Iterator<Item = &'a str>>(iter: &mut I) -> Option<Self::Output>;
}
pub trait MarkedIterScan: Sized {
    type Output;
    fn mscan<'a, I: Iterator<Item = &'a str>>(self, iter: &mut I) -> Option<Self::Output>;
}
#[derive(Debug)]
pub struct Scanner<'a> {
    iter: std::str::SplitAsciiWhitespace<'a>,
}
impl<'a> Scanner<'a> {
    #[inline]
    pub fn new(s: &'a str) -> Self {
        let iter = s.split_ascii_whitespace();
        Self { iter }
    }
    #[inline]
    pub fn scan<T: IterScan>(&mut self) -> <T as IterScan>::Output {
        T::scan(&mut self.iter).unwrap()
    }
    #[inline]
    pub fn mscan<T: MarkedIterScan>(&mut self, marker: T) -> <T as MarkedIterScan>::Output {
        marker.mscan(&mut self.iter).unwrap()
    }
    #[inline]
    pub fn scan_vec<T: IterScan>(&mut self, size: usize) -> Vec<<T as IterScan>::Output> {
        (0..size)
            .map(|_| T::scan(&mut self.iter).unwrap())
            .collect()
    }
    #[inline]
    pub fn scan_chars(&mut self) -> Vec<char> {
        self.iter.next().unwrap().chars().collect::<Vec<char>>()
    }
}
mod scanner_impls {
    use super::*;
    macro_rules !iter_scan_impls {($($t :ty ) *) =>{$(impl IterScan for $t {type Output =Self ;#[inline ] fn scan <'a ,I :Iterator <Item =&'a str >>(iter :&mut I ) ->Option <Self >{iter .next () ?.parse ::<$t >() .ok () } } ) *} ;}
    iter_scan_impls !(char u8 u16 u32 u64 usize i8 i16 i32 i64 isize f32 f64 u128 i128 String ) ;
    macro_rules !iter_scan_tuple_impl {($($T :ident ) *) =>{impl <$($T :IterScan ) ,*>IterScan for ($($T ,) *) {type Output =($(<$T as IterScan >::Output ,) *) ;#[inline ] fn scan <'a ,It :Iterator <Item =&'a str >>(_iter :&mut It ) ->Option <Self ::Output >{Some (($($T ::scan (_iter ) ?,) *) ) } } } ;}
    iter_scan_tuple_impl!();
    iter_scan_tuple_impl!(A);
    iter_scan_tuple_impl !(A B ) ;
    iter_scan_tuple_impl !(A B C ) ;
    iter_scan_tuple_impl !(A B C D ) ;
    iter_scan_tuple_impl !(A B C D E ) ;
    iter_scan_tuple_impl !(A B C D E F ) ;
    iter_scan_tuple_impl !(A B C D E F G ) ;
    iter_scan_tuple_impl !(A B C D E F G H ) ;
    iter_scan_tuple_impl !(A B C D E F G H I ) ;
    iter_scan_tuple_impl !(A B C D E F G H I J ) ;
    iter_scan_tuple_impl !(A B C D E F G H I J K ) ;
    pub struct ScannerIter<'a, 'b, T> {
        inner: &'b mut Scanner<'a>,
        _marker: std::marker::PhantomData<fn() -> T>,
    }
    impl<'a, 'b, T: IterScan> Iterator for ScannerIter<'a, 'b, T> {
        type Item = <T as IterScan>::Output;
        fn next(&mut self) -> Option<Self::Item> {
            T::scan(&mut self.inner.iter)
        }
    }
    impl<'a> Scanner<'a> {
        #[inline]
        pub fn iter<'b, T: IterScan>(&'b mut self) -> ScannerIter<'a, 'b, T> {
            ScannerIter {
                inner: self,
                _marker: std::marker::PhantomData,
            }
        }
    }
}
mod marker {
    use super::*;
    pub struct Usize1;
    impl IterScan for Usize1 {
        type Output = usize;
        #[inline]
        fn scan<'a, I: Iterator<Item = &'a str>>(iter: &mut I) -> Option<Self::Output> {
            usize::scan(iter).map(|x| x.wrapping_sub(1))
        }
    }
    pub struct Isize1;
    impl IterScan for Isize1 {
        type Output = isize;
        #[inline]
        fn scan<'a, I: Iterator<Item = &'a str>>(iter: &mut I) -> Option<Self::Output> {
            isize::scan(iter).map(|x| x.wrapping_sub(1))
        }
    }
}
#[macro_export]
macro_rules !min {($e :expr ) =>{$e } ;($e :expr ,$($es :expr ) ,+) =>{std ::cmp ::min ($e ,min !($($es ) ,+) ) } ;}
#[macro_export]
macro_rules !chmin {($dst :expr ,$($src :expr ) ,+) =>{{let x =std ::cmp ::min ($dst ,min !($($src ) ,+) ) ;$dst =x ;} } ;}
#[macro_export]
macro_rules !max {($e :expr ) =>{$e } ;($e :expr ,$($es :expr ) ,+) =>{std ::cmp ::max ($e ,max !($($es ) ,+) ) } ;}
#[macro_export]
macro_rules !chmax {($dst :expr ,$($src :expr ) ,+) =>{{let x =std ::cmp ::max ($dst ,max !($($src ) ,+) ) ;$dst =x ;} } ;}
#[derive(Clone, Debug)]
pub struct QueueAggregation<M: Monoid> {
    front_stack: Vec<(M::T, M::T)>,
    back_stack: Vec<(M::T, M::T)>,
    monoid: M,
}
impl<M: Monoid> QueueAggregation<M> {
    pub fn new(monoid: M) -> Self {
        Self {
            front_stack: Vec::new(),
            back_stack: Vec::new(),
            monoid,
        }
    }
    #[inline]
    pub fn len(&self) -> usize {
        self.front_stack.len() + self.back_stack.len()
    }
    #[inline]
    pub fn is_empty(&self) -> bool {
        self.front_stack.is_empty() && self.back_stack.is_empty()
    }
    #[inline]
    pub fn fold_all(&self) -> M::T {
        self.monoid.operate(
            self.front_stack
                .last()
                .map(|t| &t.0)
                .unwrap_or(&self.monoid.unit()),
            self.back_stack
                .last()
                .map(|t| &t.0)
                .unwrap_or(&self.monoid.unit()),
        )
    }
    #[inline]
    pub fn last(&self) -> Option<&M::T> {
        self.back_stack
            .last()
            .or(self.front_stack.first())
            .map(|t| &t.1)
    }
    #[inline]
    pub fn push(&mut self, value: M::T) {
        let x = self.monoid.operate(
            self.back_stack
                .last()
                .map(|t| &t.0)
                .unwrap_or(&self.monoid.unit()),
            &value,
        );
        self.back_stack.push((x, value));
    }
    #[inline]
    fn push_front(&mut self, value: M::T) {
        let x = self.monoid.operate(
            &value,
            self.front_stack
                .last()
                .map(|t| &t.0)
                .unwrap_or(&self.monoid.unit()),
        );
        self.front_stack.push((x, value));
    }
    #[inline]
    pub fn pop(&mut self) -> Option<M::T> {
        if self.front_stack.is_empty() {
            let v = self
                .back_stack
                .drain(..)
                .map(|t| t.1)
                .rev()
                .collect::<Vec<_>>();
            for x in v.into_iter() {
                self.push_front(x)
            }
        }
        self.front_stack.pop().map(|t| t.1)
    }
}
/// binary operaion: $T \circ T \to T$
pub trait Magma {
    /// type of operands: $T$
    type T: Clone + PartialEq;
    /// binary operaion: $\circ$
    fn operate(&self, x: &Self::T, y: &Self::T) -> Self::T;
    #[inline]
    fn reverse_operate(&self, x: &Self::T, y: &Self::T) -> Self::T {
        self.operate(x, y)
    }
}
/// $\forall a,\forall b,\forall c \in T, (a \circ b) \circ c = a \circ (b \circ c)$
pub trait Associative {}
/// associative binary operation
pub trait SemiGroup: Magma + Associative {}
impl<S: Magma + Associative> SemiGroup for S {}
/// $\exists e \in T, \forall a \in T, e \circ a = a \circ e = e$
pub trait Unital: Magma {
    /// identity element: $e$
    fn unit(&self) -> Self::T;
}
/// associative binary operation and an identity element
pub trait Monoid: SemiGroup + Unital {
    /// binary exponentiation: $x^n = x\circ\ddots\circ x$
    fn pow(&self, x: Self::T, n: usize) -> Self::T {
        let mut n = n;
        let mut res = self.unit();
        let mut base = x;
        while n > 0 {
            if n & 1 == 1 {
                res = self.operate(&res, &base);
            }
            base = self.operate(&base, &base);
            n = n >> 1;
        }
        res
    }
}
impl<M: SemiGroup + Unital> Monoid for M {}
/// $\exists e \in T, \forall a \in T, \exists b,c \in T, b \circ a = a \circ c = e$
pub trait Invertible: Magma {
    /// $a$ where $a \circ x = e$
    fn inverse(&self, x: &Self::T) -> Self::T;
}
/// short cut of right inverse binary operation
pub trait RightInvertibleMagma: Magma {
    /// right inverse binary operation: $\cdot \circ (\cdot^{-1})$
    fn rinv_operation(&self, x: &Self::T, y: &Self::T) -> Self::T;
}
/// associative binary operation and an identity element and inverse elements
pub trait Group: Monoid + Invertible {}
impl<G: Monoid + Invertible> Group for G {}
/// $\forall a,\forall b \in T, a \circ b = b \circ a$
pub trait Commutative {}
/// commutative monoid
pub trait AbelianMonoid: Monoid + Commutative {}
impl<M: Monoid + Commutative> AbelianMonoid for M {}
/// commutative group
pub trait AbelianGroup: Group + Commutative {}
impl<G: Group + Commutative> AbelianGroup for G {}
/// $\forall a \in T, a \circ a = a$
pub trait Idempotent {}
/// idempotent monoid
pub trait IdempotentMonoid: Monoid + Idempotent {}
impl<M: Monoid + Idempotent> IdempotentMonoid for M {}
/// binary operation to select larger element
#[derive(Clone, Copy, Debug, Default, Eq, Hash, Ord, PartialEq, PartialOrd)]
pub struct MaxOperation<T: Clone + Ord + MinimumBounded> {
    _marker: std::marker::PhantomData<fn() -> T>,
}
pub trait MinimumBounded {
    fn minimum() -> Self;
}
mod max_operation_impl {
    use super::*;
    macro_rules !impl_minimum_with_min {([$($wh :tt ) *] ,$t :ty ,$min :expr ) =>{impl <$($wh ) *>MinimumBounded for $t {#[inline ] fn minimum () ->Self {$min } } } ;($t :ty ,$min :expr ) =>{impl MinimumBounded for $t {#[inline ] fn minimum () ->Self {$min } } } ;}
    impl_minimum_with_min!(usize, std::usize::MIN);
    impl_minimum_with_min!(u8, std::u8::MIN);
    impl_minimum_with_min!(u16, std::u16::MIN);
    impl_minimum_with_min!(u32, std::u32::MIN);
    impl_minimum_with_min!(u64, std::u64::MIN);
    impl_minimum_with_min!(isize, std::isize::MIN);
    impl_minimum_with_min!(i8, std::i8::MIN);
    impl_minimum_with_min!(i16, std::i16::MIN);
    impl_minimum_with_min!(i32, std::i32::MIN);
    impl_minimum_with_min!(i64, std::i64::MIN);
    macro_rules !impl_minimum_tuple {($($T :ident ) *) =>{impl <$($T :MinimumBounded ) ,*>MinimumBounded for ($($T ,) *) {#[inline ] fn minimum () ->Self {($(<$T as MinimumBounded >::minimum () ,) *) } } } ;}
    impl_minimum_tuple!();
    impl_minimum_tuple!(A);
    impl_minimum_tuple !(A B ) ;
    impl_minimum_tuple !(A B C ) ;
    impl_minimum_tuple !(A B C D ) ;
    impl_minimum_tuple !(A B C D E ) ;
    impl_minimum_tuple !(A B C D E F ) ;
    impl_minimum_tuple !(A B C D E F G ) ;
    impl_minimum_tuple !(A B C D E F G H ) ;
    impl_minimum_tuple !(A B C D E F G H I ) ;
    impl_minimum_tuple !(A B C D E F G H I J ) ;
    impl_minimum_tuple !(A B C D E F G H I J K ) ;
    impl<T: Clone + Ord + MinimumBounded> MaxOperation<T> {
        pub fn new() -> Self {
            Self {
                _marker: std::marker::PhantomData,
            }
        }
    }
    impl<T: Clone + Ord + MinimumBounded> Magma for MaxOperation<T> {
        type T = T;
        #[inline]
        fn operate(&self, x: &Self::T, y: &Self::T) -> Self::T {
            std::cmp::max(x, y).clone()
        }
    }
    impl<T: Clone + Ord + MinimumBounded> Unital for MaxOperation<T> {
        #[inline]
        fn unit(&self) -> Self::T {
            MinimumBounded::minimum()
        }
    }
    impl<T: Clone + Ord + MinimumBounded> Associative for MaxOperation<T> {}
    impl<T: Clone + Ord + MinimumBounded> Commutative for MaxOperation<T> {}
    impl<T: Clone + Ord + MinimumBounded> Idempotent for MaxOperation<T> {}
}
/// $+$
#[derive(Clone, Copy, Debug, Default, Eq, Hash, Ord, PartialEq, PartialOrd)]
pub struct AdditiveOperation<T: Copy + PartialEq + AdditiveIdentity> {
    _marker: std::marker::PhantomData<fn() -> T>,
}
pub trait AdditiveIdentity: Sized + std::ops::Add<Output = Self> {
    fn zero() -> Self;
}
#[macro_use]
mod additive_operation_impl {
    use super::*;
    #[macro_export(local_inner_macros)]
    macro_rules !impl_additive_identity {([$($wh :tt ) *] ,$t :ty ,$zero :expr ) =>{impl <$($wh ) *>AdditiveIdentity for $t {#[inline ] fn zero () ->Self {$zero } } } ;($t :ty ,$zero :expr ) =>{impl AdditiveIdentity for $t {#[inline ] fn zero () ->Self {$zero } } } ;}
    impl_additive_identity!(usize, 0usize);
    impl_additive_identity!(u8, 0u8);
    impl_additive_identity!(u16, 0u16);
    impl_additive_identity!(u32, 0u32);
    impl_additive_identity!(u64, 0u64);
    impl_additive_identity!(isize, 0isize);
    impl_additive_identity!(i8, 0i8);
    impl_additive_identity!(i16, 0i16);
    impl_additive_identity!(i32, 0i32);
    impl_additive_identity!(i64, 0i64);
    impl_additive_identity!(f32, 0.0f32);
    impl_additive_identity!(f64, 0.0f64);
    impl<T: Copy + PartialEq + AdditiveIdentity> AdditiveOperation<T> {
        pub fn new() -> Self {
            Self {
                _marker: std::marker::PhantomData,
            }
        }
    }
    impl<T: Copy + PartialEq + AdditiveIdentity> Magma for AdditiveOperation<T> {
        type T = T;
        #[inline]
        fn operate(&self, x: &Self::T, y: &Self::T) -> Self::T {
            *x + *y
        }
    }
    impl<T: Copy + PartialEq + AdditiveIdentity> Unital for AdditiveOperation<T> {
        #[inline]
        fn unit(&self) -> Self::T {
            AdditiveIdentity::zero()
        }
    }
    impl<T: Copy + PartialEq + AdditiveIdentity> Associative for AdditiveOperation<T> {}
    impl<T: Copy + PartialEq + AdditiveIdentity> Commutative for AdditiveOperation<T> {}
    impl<T: Copy + PartialEq + AdditiveIdentity + std::ops::Neg<Output = T>> Invertible
        for AdditiveOperation<T>
    {
        #[inline]
        fn inverse(&self, x: &Self::T) -> Self::T {
            -*x
        }
    }
    impl<T: Copy + PartialEq + AdditiveIdentity + std::ops::Sub<Output = T>> RightInvertibleMagma
        for AdditiveOperation<T>
    {
        #[inline]
        fn rinv_operation(&self, x: &Self::T, y: &Self::T) -> Self::T {
            *x - *y
        }
    }
}
fn main() {
    #[allow(unused_imports)]
    use std::io::{Read as _, Write as _};
    let __out = std::io::stdout();
    let mut __in_buf = String::new();
    std::io::stdin().read_to_string(&mut __in_buf).unwrap();
    let mut __scanner = Scanner::new(&__in_buf);
    #[allow(unused_macros)]
    macro_rules !scan {() =>{scan !(usize ) } ;(($($t :tt ) ,*) ) =>{($(scan !($t ) ) ,*) } ;([$t :tt ;$len :expr ] ) =>{(0 ..$len ) .map (|_ |scan !($t ) ) .collect ::<Vec <_ >>() } ;({$t :tt =>$f :expr } ) =>{$f (scan !($t ) ) } ;(chars ) =>{__scanner .scan_chars () } ;($t :ty ) =>{__scanner .scan ::<$t >() } ;}
    let mut __out = std::io::BufWriter::new(__out.lock());
    #[allow(unused_macros)]
    macro_rules !print {($($arg :tt ) *) =>(::std ::write !(__out ,$($arg ) *) .unwrap () ) }
    #[allow(unused_macros)]
    macro_rules !println {($($arg :tt ) *) =>(::std ::writeln !(__out ,$($arg ) *) .unwrap () ) }
    #[allow(unused_macros)]
    macro_rules! echo {
        ($iter :expr ) => {
            echo!($iter, "\n")
        };
        ($iter :expr ,$sep :expr ) => {
            let mut iter = $iter;
            if let Some(item) = iter.next() {
                print!("{}", item);
            }
            for item in iter {
                print!("{}{}", $sep, item);
            }
            println!();
        };
    }
    let n = scan!();
    let k = scan!();
    let p = scan!([usize; n]);
    let mut q = QueueAggregation::new(AdditiveOperation::new());
    let mut ans = 0;
    for p in p {
        q.push(p + 1);
        if q.len() == k {
            chmax!(ans, q.fold_all());
            q.pop();
        }
    }
    println!("{}", ans as f64 / 2.);
}

Measured memory_bytes: 12532