#pragma GCC optimize("O3")
#pragma GCC target("avx")
#include <algorithm>
#include <array>
#include <bitset>
#include <cstdio>
#include <random>
#include <vector>
#define repeat(i, n) for (int i = 0; (i) < int(n); ++(i))
#define repeat_from(i, m, n) for (int i = (m); (i) < int(n); ++(i))
using namespace std;
template <class T> inline void setmax(T & a, T const & b) { a = max(a, b); }

constexpr int MAX_N = 500000;
char a[2 * MAX_N];
default_random_engine gen;
int main() {
    int n, k; scanf("%d%d", &n, &k);
    repeat (i, n) {
        char a_i; scanf(" %c", &a_i);
        a[i] = a[n + i] = (a_i == '1');
    }
    double result = 0;
    repeat_from (len, k, n + 1) {
        if (abs(len - k) >= 1000 and abs(n - len) >= 1000 and not bernoulli_distribution(0.001)(gen)) continue;
        int cnt = 0;
        repeat (i, len) cnt += a[i];
        int max_cnt = cnt;
        repeat (i, n) {
            cnt += - a[i] + a[i + len];
            setmax(max_cnt, cnt);
        }
        setmax(result, max_cnt /(double) len);
    }
    printf("%.15lf\n", result);
    return 0;
}