#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int day, require;
  cin >> day >> require;
  int arr[day];
  for (int i = 0; i < day; i++) cin >> arr[i];
  double now = 0, answer = 0;
  for (int i = 0; i < require - 1; i++) now += arr[i];
  for (int i = require; i <= day; i++) {
    now += arr[i - 1];
    answer = max(answer, now / i);
    double temp = now;
    for (int j = i; j < day; j++) {
      temp += arr[j] - arr[j - i];
      answer = max(answer, temp / i);
    }
  }
  cout << setprecision(15) << fixed << answer << endl;
  cerr << "\nTime elapsed: " << 1000 * clock() / CLOCKS_PER_SEC << " ms\n";
  return 0;
}
