#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  cin >> n >> k;
  double ara[n + 5];
  double sum = 0;
  ara[0] = 0;
  for (int i = 1; i <= n; i++) cin >> ara[i];
  for (int i = 1; i <= n; i++) {
    sum += ara[i];
    ara[i] = sum / 1.0;
  }
  vector<double> v;
  for (int i = 1; i <= n - k + 1; i++) {
    int y = 0;
    for (int j = i; j <= n - k + 1; j++) {
      v.push_back(((ara[j + k - 1] - ara[i - 1])) / (k + y));
      y++;
    }
  }
  sort(v.begin(), v.end());
  cout << fixed << setprecision(10) << v[v.size() - 1] << endl;
  ;
}
