#include <bits/stdc++.h>

#define Nmax 30

using namespace std;

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

int N, K;
long double dp[Nmax][Nmax];
long double P[Nmax];
int C[Nmax][Nmax];

int main()
{
    fin >> N >> K;
    dp[0][0] = 1;
    C[0][0] = 1;
    for(int i = 1; i <= N; i++)
        fin >> P[i];
    for(int i = 1; i <= N; i++)
    {
        dp[i][0] = C[i][0] = 1;
        for(int j = 1; j <= N; j++)
        {
            C[i][j] = C[i - 1][j] + C[i - 1][j - 1];
            dp[i][j] = P[i] * dp[i - 1][j - 1] + dp[i - 1][j];
        }
    }
    fout << fixed << setprecision(9) << dp[N][K] / C[N][K] << "\n";
    return 0;
}