#include <bits/stdc++.h>
using namespace std;
long long n;
vector<long long> a;
vector<double> res;
vector<long long> sum;
void temprature(int k) {
  if (k > a.size()) {
    return;
  }
  for (int i = 0; i < n; i++) {
    if (i + k - 1 < n) {
      long long temp;
      if (i - 1 >= 0)
        temp = sum[i + k - 1] - sum[i - 1];
      else
        temp = sum[i + k - 1];
      double temp1 = (temp + .0) / (k + .0);
      res.push_back(temp1);
    }
  }
  temprature(k + 1);
}
int main() {
  int k;
  cin >> n >> k;
  a.resize(n);
  sum.resize(n);
  for (int i = 0; i < n; i++) {
    cin >> a[i];
    if (i > 0)
      sum[i] = sum[i - 1] + a[i];
    else
      sum[i] = a[i];
  }
  temprature(k);
  sort(res.rbegin(), res.rend());
  cout << setprecision(7) << fixed;
  cout << res[0];
  return 0;
}
