#[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
}

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

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

impl<T> std::cmp::Eq for Ordered<T>
where
    T: std::cmp::PartialOrd,
{}

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

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();
    let check = |x| {
        // debug!(x);
        let mut set = VecDeque::new();
        set.push_back(0.);
        for &e in &a {
            let b = *set.back().unwrap();
            set.push_back(b + e as f64 - x);
        }
        // debug!(set);
        let mut os = BTreeMap::new();
        for e in set.iter().skip(k).cloned().map(Ordered) {
            *os.entry(e).or_insert(0) += 1;
        }
        
        for &e in &a {
            // debug!(set);
            // debug!(os);
            {
                let (Ordered(min), _) = os.iter().rev().next().unwrap();
                let min = *min;


                // debug!((min, *set.front().unwrap()));
                if min > *set.front().unwrap() {
                    return false;
                }
            }
            // os.remove(&Ordered(*set.get(k).unwrap()));
            *os.entry(Ordered(*set.get(k).unwrap())).or_insert(-1) -= 1;
            if *os.get(&Ordered(*set.get(k).unwrap())).unwrap() == 0 {
                os.remove(&Ordered(*set.get(k).unwrap()));
            } 
            // debug!(os);
            set.pop_front();
            let b = *set.back().unwrap();
            // os.insert(Ordered(b + e as f64 - x));
            *os.entry(Ordered(b + e as f64 - x)).or_insert(0) += 1;
            // debug!(os);
            set.push_back(b + e as f64 - x);
        }
        true
    };
    // for e in 0..100 {
    //     let mut a = 0.01 * e as f64;
    //     debug!(check(a));
    // }
    // debug!(check(0.9));
    let mut l = 0.;
    let mut r = 1.;
    for _ in 0..50 {
        let m = (l+r)/2.;
        let res = check(m);
        // debug!(res);
        if res {
            r = m;
        } else {
            l = m;
        }
    }
    printfln!("{}", r);
}