#include <iostream>
#include <fstream>
#include <vector>
#include <iomanip>

using namespace std;

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

int n, k;

vector <double> prob(26);
vector <int> viz(26);
vector <int> v(26);
double answ = 0;
double rezprod = 1;

int main()
{
    f >> n >> k;

    for (int i = 1; i <= n; i++)
        f >> prob[i];

    vector <vector <double>> dp(n + 2, vector <double>(k + 2, 0));


    for (int i = 0; i <= n; i++)
        dp[i][0] = 1.0;

    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= k; j++)
            {
                dp[i][j] = dp[i - 1][j] + dp[i - 1][j - 1] * prob[i];
            }

    answ = dp[n][k];

   // cout << answ;

    for (int i = 1; i <= n; i++)
    {
        if (i >= k + 1)
            answ = answ / i;
        if (i <= n - k)
            answ *= i;
    }

    g << setprecision(6) << answ;
    return 0;
}