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

using namespace std;

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

void combinari(int idx, int n, int k, vector <double> &p, vector <double> &chosen, double &sol, int &num_cmb) {
    if (chosen.size() == k) {
        double prod = 1;
        for (auto &&pr: chosen) {
            prod *= pr;
        }
        sol += prod;
        num_cmb++;
        return;
    }

    for (int i = idx; i < n; i++) {
        chosen.push_back(p[i]);
        combinari(i + 1, n, k, p, chosen, sol, num_cmb);
        chosen.pop_back();
    }
}

int main() {
    int n, k;
    in >> n >> k;
    vector <double> p(n);
    vector <double> chosen;

    for (int i = 0; i < n; i++) {
        in >> p[i];
    }

    int num_cmb = 0;
    double sol = 0;
    combinari(0, n, k, p, chosen, sol, num_cmb);
    out << fixed << setprecision(10) << sol / num_cmb << '\n';
    return 0;
}