#include <bits/stdc++.h>
using namespace std;
double dp[100000];
int main() {
  int n, k;
  cin >> n >> k;
  double ar[n];
  for (int i = 0; i < n; i++) {
    scanf("%lf", &ar[i]);
  }
  for (int i = k; i <= n; i++) {
    double temp = 0.0;
    for (int j = 0; j < i; j++) {
      temp += ar[j];
    }
    dp[i] = max(dp[i], temp);
    for (int j = 0; j + i < n; j++) {
      temp -= ar[j];
      temp += ar[j + i];
      dp[i] = max(dp[i], temp);
    }
  }
  double ans = 0.0;
  for (int i = k; i <= n; i++) {
    ans = max(ans, dp[i] / (double)i);
  }
  printf("%.13lf", ans);
}
