Codeforces solution for 808B

808b.cpp

#include <iostream>
#include <cstdio>
using namespace std;

int main() {
  int n, k;
  cin >> n >> k;
  
  int sum = 0, m;
  
  cin >> m;
  sum += m;
  
  if (n == 1) {
    printf("%.10f\n", (double) sum);
    return 0;
  }
  
  for (int i = 1; i < n - 1; i++) {
    cin >> m;
    sum += m * 2;
  }
  
  cin >> m;
  sum += m;

  double d = (double) sum / (n - k + 1);
  printf("%.10f\n", d);
}