#include <bits/stdc++.h>
using namespace std;
const int maxn = 5000 + 10;
int main() {
  int n, k;
  int a[maxn];
  scanf("%d%d", &n, &k);
  for (int i = 0; i < n; i++) scanf("%d", &a[i]);
  int sum = 0;
  int maxs = 0;
  double maxa = 0;
  for (int i = k; i <= n; i++) {
    for (int j = 0; j < i; j++) sum += a[j];
    maxs = sum;
    for (int j = 1; j < n - i + 1; j++) {
      sum = sum - a[j - 1] + a[j + i - 1];
      maxs = max(maxs, sum);
    }
    double a = maxs * 1.0 / i;
    maxa = max(maxa, a);
    sum = 0;
  }
  printf("%lf\n", maxa);
  return 0;
}
