#include <fstream>
#include <iomanip>
#define nmax 25
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
int n, k, num;
float p[nmax], p_tot;

void bkt(int r, float p_, int lst)
{
    int i;
    if (r <= k)
        for (i = lst + 1; i <= n; i++)
                bkt(r + 1, p_ * p[i], i);
    else {
        p_tot += p_;
        num ++;
    }
}
int main()
{
    int i, j;
    f >> n >> k;
    for (i = 1; i <= n; i++)
        f >> p[i];

    bkt(1, 1.0, 0);
    p_tot /= num;
    g << setprecision(6) << fixed << p_tot << '\n';

    return 0;
}