#include <iostream>
#include <vector>
#include <fstream>
#include <cmath>
#include <iomanip>
using namespace std;
ifstream in ("dezastru.in");
ofstream out ("dezastru.out");
int64_t n, k;
vector <double> pr (26);
int rz [26];
bool used[26];
double rs = 0;

void subsets()
{
    for (int i = 0; i < (1 << n); i++){
        int count = 0;
        double temp = 1;
        for (int j = 0; j < n; j++){
            if ((i & (1 << j)) != 0){
                temp *= pr[j+1];
               //cout << j+1 << " ";
                count++;
            }
        }
        if (count == k){
            rs+= temp;
            //cout << rs << "\n";
            //cout  << "\n" << temp << "\n";
            //cout << "\n";
        }
    }
}
	
int64_t fastcomb(int64_t n, int64_t k) {
	int64_t mn = min(k, n - k);
	int64_t mx = max(k, n - k);
 
	int64_t ans = 1;
	for (int i = mx + 1; i <= n; i++) {
		ans *= i;
	}
 
	for (int i = 1; i <= mn; i++) {
		ans /= i;
	}
 
	return ans;
}

int main()
{
    in >> n >> k;
    for (int i = 1; i <= n; i++){
        in >> pr[i];
    }
    double fact1 = 1;
    double fact2 = 1;
    subsets();

  /*  if (k > n-k){
        for (int i = k+1; i <=n; i++)
            fact1 *=i;
        for (int i = 2; i <= n-k; i++)
            fact2 *= i;
    }
    else{
        for (int i = n-k+1; i <=n; i++){
            fact1 *=i;
        }
        for (int i = 2; i <= k; i++){
            fact2 *=i;
        }
    }*/
    double fact3 = fact1 / fact2;
    out << fixed << setprecision(6) << rs / fastcomb(n,k);
    return 0;
}