#include <iostream>
#include <fstream>
#include <cstdio>
#include <cmath>
#include <vector>
#include <cstring>
#include <string>
#include <set>
#include <map>
#include <stack>
#include <queue>
#include <deque>
#include <algorithm>
#include <random>
using namespace std;

#define REP(i,n) for(int i=0; i<n; ++i)
#define FOR(i,a,b) for(int i=a; i<=b; ++i)
#define FORR(i,a,b) for (int i=a; i>=b; --i)
#define ALL(c) (c).begin(), (c).end()

typedef long long ll;
typedef vector<int> VI;
typedef vector<ll> VL;
typedef vector<VL> VVL;
typedef vector<VI> VVI;
typedef pair<int,int> P;
typedef pair<ll,ll> PL;

void slide_min(vector<double> &a, int k, vector<double> & ret){
    deque<double> dq;
    REP(i,a.size()){
        while (!dq.empty() && a[dq.back()] >= a[i]) dq.pop_back();
        dq.push_back(i);
        if (i - k + 1 >= 0){
            ret.push_back(a[dq.front()]);
            if (dq.front() == i - k + 1) dq.pop_front();
        }
    }
}

bool check(vector<double> &a, int k){
    int n = a.size() / 2;
    vector<double> b(2*n+1);
    REP(i,2*n) b[i+1] += b[i] + a[i];
    vector<double> mi;
    slide_min(b, n - k + 1, mi);
    // REP(i,2*n+1) cout << b[i] << " ";
    // cout << endl;
    // REP(i,n) cout << mi[i] << " ";
    // cout << endl;
    REP(i,n) if (mi[i] <= b[n+i]) return true;
    return false;
}

int main() {
    int n, k;
    string s;
    cin >> n >> k;
    cin >> s;

    // cout << 0 << endl;
    // return 0;

    vector<double> a(n);
    REP(i,n) a[i] = s[i] - '0';
    REP(i,n) a.push_back(a[i]);

    double ok = 0.0, ng = 1.0;
    REP(_,50){
        vector<double> b(a);
        double mi = (ok + ng) / 2.0;
        REP(i,2*n) b[i] -= mi;
        if (check(b, k)) ok = mi;
        else ng = mi;
    }
    printf("%.10f\n", ok);

    return 0;
}