#include <fstream>
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
int n, k;
float arr[30], F[30], DP[30][30];
void Solve() {
    F[0] = 1;
    for (int i = 1; i <= n; i++)
        F[i] = i * F[i-1];
    DP[0][0] = 1;
    for (int i = 1; i <= n; i++) {
        DP[i][0] = 1;
        for (int j = 1; j <= i; j++)
            DP[i][j] = DP[i-1][j] + DP[i-1][j-1]*arr[i];
    }
    g<< DP[n][k]*F[k]*F[n-k]/F[n];
}

int main() {
    f>>n>> k;
    for (int i = 1; i <= n; i++)
        f>> arr[i];

    Solve();
}