#include <bits/stdc++.h>

using namespace std;

int main() {
  ifstream cin("dezastru.in");
  ofstream cout("dezastru.out");
  double n, k;
  cin >> n >> k;
  vector<double> v(n);
  for (int i = 0; i < n; i++)
    cin >> v[i];
  long double res = 0;
  do {
    long double r = 1;
    for (int i = 0; i < k; i++)
      r *= v[i];
    res += r / (n * 2);
  } while (next_permutation(v.begin(), v.end()));
  cout << setprecision(6) << fixed << res;
}