#include <iostream>
#include <fstream>
#include <iomanip>
#define NMAX 30
using namespace std;
ifstream fin("dezastru.in");
ofstream fout("dezastru.out");

int n, k;
double p[NMAX];
double dp[NMAX][NMAX];

double fact(int nr)
{
    int prod = 1;
    for (int i = 1; i <= nr; i++)
        prod *= i;
    return prod;
}

int main()
{
    fin >> n >> k;
    dp[0][0] = 1;
    for (int i = 1; i <= n; i++)
    {
        fin >> p[i];
        dp[i][0] = 1;
    }
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= i; j++)
            dp[i][j] = dp[i - 1][j] + p[i] * dp[i - 1][j - 1];

    fout << fixed << setprecision(7) << dp[n][k] / fact(n) * fact(n - k) * fact(k)<< "\n";
    return 0;
}