#include <bits/stdc++.h>
using namespace std;
long double cs[5005];
int main() {
  long long n, k, sum = 0;
  cin >> n >> k;
  vector<long long> v(n);
  for (int i = 0; i < n; i++) {
    cin >> v[i];
    sum += v[i];
    cs[i] = sum;
  }
  long double ma = cs[k - 1];
  long long cnt = 0, j;
  long double ans = 0.0;
  for (int i = 0; i < n; i++) {
    if (i - cnt >= k - 1) {
      j = i;
      while (j < n) {
        if (cnt > 0) {
          ans = max(ans, (cs[j] - cs[cnt - 1]) / (long double)(k + j - i));
        } else {
          ans = max(ans, cs[j] / (long double)(k + j - i));
        }
        j++;
      }
      cnt++;
    }
  }
  cout << setprecision(10) << ans << endl;
}
