#include <bits/stdc++.h>
using namespace std;
int n, k, l, r;
long long s, smax, a[5005];
int main() {
  cin >> n >> k;
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
    a[i] = a[i] + a[i - 1];
  }
  smax = a[k];
  l = 1;
  r = k;
  for (int i = 1; i <= n - k + 1; i++)
    for (int j = i + k - 1; j <= n; j++) {
      if (smax * (j - i + 1) < (a[j] - a[i - 1]) * (r - l + 1)) {
        smax = a[j] - a[i - 1];
        l = i;
        r = j;
      }
    }
  double ans = smax;
  cout << setprecision(10) << fixed << ans / (r - l + 1);
  return 0;
}
