#include <bits/stdc++.h>
using namespace std;
int n, k;
double a[5005], sum[5005], maxx;
int main() {
  cin >> n >> k;
  for (int i = 1; i <= n; i++) cin >> a[i];
  for (int i = 1; i <= n; i++) sum[i] = sum[i - 1] + a[i];
  for (int i = k; i <= n; i++) {
    for (int j = 1; j <= n - i + 1; j++) {
      double ans = (sum[j + i - 1] - sum[j - 1]) / i;
      if (ans > maxx) maxx = ans;
    }
  }
  printf("%.7f", maxx);
  return 0;
}
