#[allow(dead_code)]
fn getline() -> String {
    let mut res = String::new();
    std::io::stdin().read_line(&mut res).ok();
    res
}

#[allow(unused_macros)]
macro_rules! readl {
    ($t: ty) => {
        {
            let s = getline();
            s.trim().parse::<$t>().unwrap()
        }
    };
    ($( $t: ty),+ ) => {
        {
            let s = getline();
            let mut iter = s.trim().split(' ');
            ($(iter.next().unwrap().parse::<$t>().unwrap(),)*)
        }
    };
}

#[allow(unused_macros)]
macro_rules! readlvec {
    ($t:ty) => {{
        let s = getline();
        let iter = s.trim().split(' ');
        iter.map(|x| x.parse().unwrap()).collect::<Vec<$t>>()
    }};
}

#[allow(unused_macros)]
// macro_rules! debug {
//     ($x:expr) => {
//         println!("{}: {:?}", stringify!($x), $x)
//     };
// }
macro_rules! debug { ($x: expr) => () }

#[allow(dead_code)]
fn show<T>(iter: T) -> String
where
    T: Iterator,
    T::Item: std::fmt::Display,
{
    let mut res = iter.fold(String::new(), |sum, e| sum + &format!("{} ", e));
    res.pop();
    res
}

fn sliding_minimum<T>(a: &Vec<T>, width: usize) -> Vec<T>
where
    T: std::cmp::PartialOrd + Clone
{
    let mut res = Vec::with_capacity(a.len() - width);
    let mut set = std::collections::VecDeque::new();

    for (i, e) in a.iter().enumerate().take(width) {
        while let Some((v, i)) = set.pop_back() {
            if v < e {
                set.push_back((v, i));
                break;
            }
        }
        set.push_back((e, i));
    }
    for (s, e) in a.iter().skip(width).enumerate() {
        res.push(set.front().expect("sliding_minimum error").0.clone());
        while let Some((v, i)) = set.pop_back() {
            if v < e {
                set.push_back((v, i));
                break;
            }
        }
        set.push_back((e, s + width));
        while let Some((v, i)) = set.pop_front() {
            if s + 1 <= i {
                set.push_front((v, i));
                break;
            }
        }
    }
    res.push(set.front().expect("sliding_minimum error").0.clone());

    res
}

#[allow(unused_imports)]
use std::cmp::{max, min};
#[allow(unused_imports)]
use std::collections::btree_map::Entry::{Occupied, Vacant};
#[allow(unused_imports)]
use std::collections::{BTreeMap, BTreeSet, BinaryHeap, HashMap, HashSet, VecDeque};

#[derive(PartialEq, PartialOrd, Debug, Clone)]
struct Ordered<T: PartialOrd>(T);

impl<T> Eq for Ordered<T>
where
    T: PartialOrd + Clone,
{
}

impl<T> Ord for Ordered<T>
where
    T: PartialOrd + Clone,
{
    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
        self.partial_cmp(other).unwrap()
    }
}

impl<T: Copy + PartialOrd> Copy for Ordered<T> {}

impl<T: PartialOrd> Ordered<T> {
    fn unwrap(self) -> T {
        let Ordered(res) = self;
        res
    }
}

#[derive(Eq, PartialEq, Clone, Debug)]
struct Reverse<T>(T);

impl<T: PartialOrd> PartialOrd for Reverse<T> {
    fn partial_cmp(&self, other: &Reverse<T>) -> Option<std::cmp::Ordering> {
        other.0.partial_cmp(&self.0)
    }
}

impl<T: Ord> Ord for Reverse<T> {
    fn cmp(&self, other: &Reverse<T>) -> std::cmp::Ordering {
        other.0.cmp(&self.0)
    }
}


fn main() {
    use std::io::Write;
    let out = std::io::stdout();
    let mut out = std::io::BufWriter::new(out.lock());
    macro_rules! printf { ($($arg:tt)*) => (write!(out, $($arg)*).unwrap()); }
    macro_rules! printfln { () => (writeln!(out).unwrap()); ($($arg:tt)*) => (writeln!(out, $($arg)*).unwrap()); }

    let (n, k) = readl!(usize, usize);
    let a: Vec<i32> = readl!(String)
        .into_bytes()
        .into_iter()
        .map(|x| if x == b'0' { 0 } else { 1 })
        .collect();
    
    
    
    if n == 1 {
        printfln!("{}", a[0]);
        return;
    }
    // let check = |x: f64| {
    //     let mut a: Vec<_> = vec![Reverse(Ordered(0.))].into_iter()
    //         .chain(
    //             a.iter().cloned()
    //             .chain(a.iter().cloned())
    //             .map(|e| Reverse(Ordered(e as f64 - x)))
    //         ).collect();
    //     for i in 0..a.len() - 1 {
    //         (a[i + 1].0).0 += (a[i].0).0;
    //     }
    //     let maxi: Vec<_> = sliding_minimum(
    //         &a,
    //         n - k,
    //     ).into_iter().map(|Reverse(Ordered(x))| x).collect();
    //     for (Reverse(Ordered(l)), &rmax) in a.into_iter().take(n+1).zip(maxi.iter().cycle().skip(k)) { 
    //         if l < rmax {
    //             return false;
    //         }
    //     }
    //     true
    // };
    // let check = |h| {
    //     let mut min = -1e15;
    //     let mut cur = 0;
    //     let cum = vec![0; n*2+1];
    //     for (i, j) in 
    //     true
    // };
    let a: Vec<_> = a.iter().cloned().chain(a.iter().cloned()).collect();
    let check = |x| {
        let mut r = 0.;
        let mut l = 0.;
        let mut minl = 1e99;
        let mut li = 0;
        for ri in 0..n*2 {
            while li + k <= ri {
                if minl > l {
                    minl = l;
                }
                l += a[li] as f64 - x;
                li += 1;
            }
            if r > minl {
                return false;
            }
            r += a[ri] as f64 - x;
        }
        true
    };
    let mut l = 0.;
    let mut r = 1.;
    for _ in 0..30 {
        let m = (l + r) / 2.;
        let res = check(m);
        // debug!(res);
        if res {
            r = m;
        } else {
            l = m;
        }
    }

    printfln!("{}", (l+r)/2.);
}