#include <bits/stdc++.h>
using namespace std;
const int maxn = 2e5 + 5;
int n, k;
double a[5010];
double sum[5010];
int main() {
  cin >> n >> k;
  double ans = 0;
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
    sum[i] = sum[i - 1] + a[i];
  }
  for (int i = k; i <= n; i++) {
    for (int j = 1; j + i - 1 <= n; j++) {
      ans = max(ans, (sum[j + i - 1] - sum[j - 1]) / (double)i);
    }
  }
  printf("%.8f", ans);
  return 0;
}
