#include <bits/stdc++.h>
using namespace std;
const int maxn = 5010;
int a[maxn], sum[maxn];
int main() {
  int n, k;
  cin >> n >> k;
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
    sum[i] = a[i] + sum[i - 1];
  }
  double maxx = 0.0;
  for (int i = k; i <= n; i++) {
    for (int j = 1; j <= n - k + 1; j++) {
      double tmp = (sum[j + i - 1] - sum[j - 1]) / (double)i;
      if (tmp > maxx) maxx = tmp;
    }
  }
  printf("%.10f\n", maxx);
  return 0;
}
