#include <bits/stdc++.h>
using namespace std;

bool is_valid(double mid, int K, int N, const vector<double> & As){
    vector<double> Bs(2 * N, 0);
    for (int i = 0; i < N; ++i) Bs[i] = As[i] - mid;
    for (int i = 0; i < N; ++i) Bs[N + i] = Bs[i];
    vector<double> Cs(2 * N + 1, 0);
    for (int i = 0; i < 2 * N; ++i) Cs[i + 1] = Cs[i] + Bs[i];
    deque<pair<double, int>> window;
    for (int i = K; i <= N; ++i){
        while ((not window.empty()) and window.back().first < Cs[i]) window.pop_back();
        window.emplace_back(Cs[i], i);
    }
    if (window.front().first >= Cs[0]) return true;
    for (int i = 1; i < N; ++i){
        if (window.front().second < K + i) window.pop_front();
        while ((not window.empty()) and window.back().first < Cs[i + N]) window.pop_back();
        window.emplace_back(Cs[i + N], i + N);
        if (window.front().first >= Cs[i]) return true;
    }
    return false;
}


double solve(int K, int N, const vector<double> & As){
    double eps = 1e-7;
    double OK = 0.0;
    double NG = 1.0 + eps;
    while (OK + eps < NG){
        double mid = (OK + NG)/2.0;
        if (is_valid(mid, K, N, As)){
            OK = mid;
        }else{
            NG = mid;
        }
    }
    return OK;
}


int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    int N, K;
    cin >> N >> K;
    string S;
    cin >> S;
    vector<double> As(N, 0);
    for (int i = 0; i < N; ++i) As[i] = (double)(S[i] - '0');
    cout.precision(8);
    cout << fixed << solve(K, N, As) << endl;
    return 0;
}