#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  cin >> n >> k;
  int arr[n + 1];
  arr[0] = 0;
  for (int i = 1; i < n + 1; i++) cin >> arr[i];
  for (int i = 2; i < n + 1; i++) {
    arr[i] += arr[i - 1];
  }
  double sum = 0;
  double temp;
  for (int i = k; i < n + 1; i++) {
    for (int j = 1; j <= n; j++) {
      if (j + i - 1 <= n) {
        temp = ((double)arr[i + j - 1] - (double)arr[j - 1]) / (double)i;
        if (temp > sum) {
          sum = temp;
        }
      }
    }
  }
  cout << setprecision(15) << sum << endl;
}
