#include <fstream>

using namespace std;

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

#define n 26

int N, K;
double p[n], dp[n][n];

long double solve (int nr)
{
    long double rez = 1;
    for (int i = 1; i <= nr; i++)
    {
        rez *= i;
    }
    return rez;
}

int main()
{
    int i, j;

    in >> N >> K;
    for (i = 1; i <= N; i++)
    {
        in >> p[i];
        dp[i][0] = 1;
    }
    dp[0][0] = 1;
    for (i = 1; i <= N; i++)
    {
        for (j = 1; j <= i; j++)
        {
            dp[i][j] = dp[i - 1][j - 1] * p[i] + dp[i - 1][j];
        }
    }
    long double combinari = solve (N)/(solve (N - K) * solve (K));
    out.precision(6);
    out << dp[N][K] / combinari;
    return 0;
}