#include <algorithm>
#include <fstream>

int main() {
  std::ifstream fin("dezastru.in");
  std::ofstream fout("dezastru.out");
  int n, k;
  fin >> n >> k;
  double p[2 + n];
  for (int i = 1; i <= n; i++)
    fin >> p[i];
  double sum = 0;
  do {
    double prod = 1;
    for (int i = 1; i <= k; i++)
      prod *= p[i];
    sum += prod;
  } while (std::next_permutation(p + 1, p + n + 1));
  for (int i = 1; i <= n; i++)
    sum /= i;
  fout << sum;
  return 0;
}