#include <bits/stdc++.h>
using namespace std;
const int maxn = 5e3 + 5;
int main() {
  int n, k;
  cin >> n >> k;
  double a[maxn], f[maxn];
  a[0] = 0;
  f[0] = 0;
  for (int i = 1; i <= n; i++) {
    scanf("%lf", &a[i]);
    f[i] = f[i - 1] + a[i];
  }
  double maxx = 0;
  for (int i = k; i <= n; i++) {
    for (int j = i - k; j >= 0; j--) {
      double arv = 0;
      arv = (f[i] - f[j]) / (i - j);
      if (arv > maxx) maxx = arv;
    }
  }
  printf("%.10f\n", maxx);
  return 0;
}
