#include <fstream>

using namespace std;

ifstream fin  ("dezastru.in");
ofstream fout ("dezastru.out");

int n, k, c;
double v[50], a[50][50], sol;

inline int comb (int a, int b){
    if (a == b)
        return 1;
    if (b == 0)
        return 1;
    return comb (a - 1, b) + comb (a - 1, b - 1);
}

int main(){
    fin >> n >> k;
    a[0][0] = 1;
    for (int i=1; i<=n; i++){
        fin >> v[i];
        a[i][0] = 1;
    }
    for (int i=1; i<=n; i++){
        for (int j=1; j<=k; j++){
            a[i][j] = a[i-1][j] + a[i-1][j-1]*v[i];
        }
    }
    sol = a[n][k]/comb (n, k);
    fout << sol;
    return 0;
}