#include <fstream>
#include <iomanip>
using namespace std;
ifstream fin("dezastru.in");
ofstream fout("dezastru.out");
int N, K, Comb[30][30];
double sum, Pb[30], Dp[30][30];


int main()
{
    fin >> N >> K;
    for (int i=1; i<=N; i++) fin >> Pb[i];

    Comb[0][0] = Dp[0][0] = 1;
    for (int i=1; i<=N; i++)
    {
        Comb[i][0] = Dp[i][0] = 1;
        for (int j=1; j<=K; j++)
        {
            Dp[i][j] = Dp[i-1][j] + Dp[i-1][j-1] * Pb[i];
            Comb[i][j] = Comb[i-1][j] + Comb[i-1][j-1];
        }
    }

    fout << setprecision(6) << fixed << Dp[N][K] / Comb[N][K] << '\n';
    fout.close();
    return 0;
}