#include <bits/stdc++.h>
using namespace std;
int main(void) {
  unsigned n, k;
  cin >> n >> k;
  unsigned *temp = new unsigned[n];
  for (unsigned i = 0; i < n; ++i) {
    cin >> temp[i];
  }
  double max = 0.;
  for (int i = 0; i <= n - k; ++i) {
    unsigned sum = 0U;
    int j = i;
    for (; j < i + k; ++j) {
      sum += temp[j];
    }
    double tmp = double(sum) / (j - i);
    if (tmp > max) max = tmp;
    for (; j < n; ++j) {
      sum += temp[j];
      tmp = double(sum) / (j - i + 1);
      if (tmp > max) max = tmp;
    }
  }
  printf("%lf\n", max);
  delete[] temp;
  return 0;
}
