#include <fstream>

using namespace std;

ifstream cin ("dezastru.in");
ofstream cout ("dezastru.out");

int n, k, cnt;
double ans;

double p[30];

void bkt(int niv, double prod, int lst) {
  if(niv == k + 1) {
    ans += prod;
    cnt++;
    return;
  }
  for(int i = lst + 1; i <= n - k + niv; i++) {
    prod *= p[i];
    bkt(niv + 1, prod, i);
    prod /= p[i];
  }
}

int main() {
  cin >> n >> k;
  for(int i = 1; i <= n; i++)
    cin >> p[i];
  bkt(1, 1, 0);
  cout << ans / cnt;
  return 0;
}