#include "bits/stdc++.h"
using namespace std;
#define FOR(i,j,k) for(int (i)=(j);(i)<(int)(k);++(i))
#define rep(i,j) FOR(i,0,j)
#define each(x,y) for(auto &(x):(y))
#define mp make_pair
#define mt make_tuple
#define all(x) (x).begin(),(x).end()
#define debug(x) cout<<#x<<": "<<(x)<<endl
#define smax(x,y) (x)=max((x),(y))
#define smin(x,y) (x)=min((x),(y))
#define MEM(x,y) memset((x),(y),sizeof (x))
#define sz(x) (int)(x).size()
#define pb push_back
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<ll> vll;

vector<double> maxSlide(int length, vector<double> data, double minValue = -1) {
    int num = (int)data.size();
    deque<int> deq;
    vector<double> res(num, minValue);
    for (int i = 0; i < num; ++i) {
        while (!deq.empty() && data[deq.back()] <= data[i]) {
            deq.pop_back();
        }
        deq.push_back(i);
        if (i >= length - 1) {
            res[i - length + 1] = data[deq.front()];
            if (deq.front() == i - length + 1)deq.pop_front();
        }
    }
    return res;
}

int N, K, color[500005];

int check(double x) {
    vector<double> dat(N * 2);
    rep(i, N) {
        dat[i] = dat[i + N] = color[i] - x;
    }
    rep(i, 2 * N - 1)dat[i + 1] += dat[i];

    auto ma = maxSlide(N - K + 1, dat, INT_MIN);
    double pre = 0;
    rep(i, N) {
        if (ma[i + K - 1] >= pre)return 1;
        pre = dat[i];
    }
    return 0;
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    string S;
    cin >> N >> K >> S;
    rep(i, N)color[i] = S[i] - '0';
    
    double l = 0, r = 1, x;
    rep(ITER, 20) {
        x = (l + r)*0.5;
        (check(x) ? l : r) = x;
    }
    cout << fixed << setprecision(20);
    cout << l << endl;
}