#include <bits/stdc++.h>
using namespace std;
int a[6000];
int main() {
  int n, ks;
  cin >> n >> ks;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  double sum = 0;
  double maxs = 0;
  double rec = 0;
  for (int k = ks; k <= n; k++) {
    sum = 0;
    for (int i = 0; i < k; i++) {
      sum += a[i];
    }
    maxs = sum;
    if (k != n) {
      for (int i = k; i < n; i++) {
        sum += a[i];
        sum -= a[i - k];
        maxs = max(maxs, sum);
      }
    }
    rec = max(rec, maxs / k);
  }
  printf("%.14f", rec);
}
