#include<bits/stdc++.h>
#define rep(i,a,b) for(int i=a;i<b;i++)
#define rrep(i,a,b) for(int i=a;i>=b;i--)
#define fore(i,a) for(auto &i:a)
#pragma GCC optimize ("-O3")
using namespace std; void _main(); int main() { cin.tie(0); ios::sync_with_stdio(false); _main(); }
//---------------------------------------------------------------------------------------------------
template <class T> vector<int> slideMinimum(const vector<T> &a, int k) {
    deque<int> deq;vector<int> res;
    rep(i,0,a.size()){while(deq.size()&&a[deq.back()]>=a[i])deq.pop_back();deq.push_back(i);
    if (i-k+1>=0){res.push_back(deq.front());if(deq.front()==i-k+1)deq.pop_front();}}return res;}
/*---------------------------------------------------------------------------------------------------
　　　　　　　　　　　 ∧＿∧  
　　　　　 ∧＿∧ 　（´<_｀ ）　 Welcome to My Coding Space!
　　　　 （ ´_ゝ`）　/　 ⌒i     
　　　　／　　　＼　 　  |　|     
　　　 /　　 /￣￣￣￣/　　|  
　 ＿_(__ﾆつ/　    ＿/ .| .|＿＿＿＿  
　 　　　＼/＿＿＿＿/　（u　⊃  
---------------------------------------------------------------------------------------------------*/



#define EPS 1e-10
int N, K; string A;
//---------------------------------------------------------------------------------------------------
double B[1010101], C[1010101];
int check(double p) {
    rep(i, 0, N * 2) B[i] = (double)(A[i] - '0') - p;
    C[0] = B[0];
    rep(i, 1, N * 2) C[i] = C[i - 1] + B[i];
    rep(i, 0, N * 2) C[i] *= -1;
    auto D = slideMinimum(vector<double>(C, C + N * 2), N - K + 1);
    rep(i, 0, N * 2) C[i] *= -1;

    rep(i, 0, N) if (C[i] <= C[D[i + K]] + EPS) return 1;
    return 0;
}
//---------------------------------------------------------------------------------------------------
void _main() {
    cin >> N >> K >> A;
    A += A;

    double ok = 0, ng = 2;
    rep(i, 0, 50) {
        double x = (ok + ng) / 2;
        if (check(x)) ok = x;
        else ng = x;
    }
    printf("%.10f\n", ok);
}