/// VARIANTA 2
#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;

ifstream f ("dezastru.in");
ofstream g ("dezastru.out");

const int NMAX = 25;
int n, K;
double D[NMAX+1][NMAX+1], P[NMAX+1];

int comb(int n, int k) {
    int c = 1;
    if (n - k < k) k = n-k;
    for (int i=1; i<=k; i++)
        c = c * (n - k + i) / i;
    return c;
}

void calcul() {
    for (int i=0; i<=n; i++)
        D[i][0] = 1;
    for (int i=1; i<=n; i++)
        for (int j=1; j <= min(i,K); j++)
            D[i][j] = D[i-1][j] + D[i-1][j-1] * P[i];
}

int main()
{
    f >> n >> K;
    for (int i=1; i<=n; i++)
        f >> P[i];
    calcul();
    g << fixed << setprecision(6) << D[n][K] / comb(n, K);;
    f.close();
    g.close();
    return 0;
}