#include <bits/stdc++.h>
using namespace std;
const int MAXN = 5040;
const double EPS = 1e-9;
double cnt[MAXN];
int main() {
  int n, k;
  scanf("%d%d", &n, &k);
  for (int i = 1; i <= n; ++i) {
    scanf("%lf", &cnt[i]);
    cnt[i] += cnt[i - 1];
  }
  double ans = -1;
  for (int i = k; i <= n; ++i) {
    for (int j = 1; i + j - 1 <= n; ++j) {
      double temp = (cnt[j + i - 1] - cnt[j - 1]) / i;
      if (ans - temp < EPS) ans = temp;
    }
  }
  printf("%.15f\n", ans);
  return 0;
}
