#include <fstream>
#include <iomanip>

using namespace std;

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

double p[30], a[30][30];

int main()
{
    int n, k;
    fin >> n >> k;
    for(int i = 1; i <= n; ++i)
    {
        fin >> p[i];
        a[i][0] = 1;
    }
    fin.close();

    a[0][0] = 1;
    for(int i = 1; i <= n; ++i)
        for(int j = 1; j <= i; ++j) a[i][j] = a[i-1][j-1] * p[i] + a[i-1][j];
    double comb = 1;
    for(int i = k + 1; i <= n; ++i) comb *= i;
    for(int i = 2; i <= n - k; ++i) comb /= i;

    fout << setprecision(6) << a[n][k] / comb;
    fout.close();
    return 0;
}