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

using namespace std;

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

int main() {
    int n, k;
    in >> n >> k;
    vector <double> p(n);
    for (int i = 0; i < n; i++) {
        in >> p[i];
    }

    vector <vector <double> > dp(n + 1, vector <double> (k + 1, 0.0));
    vector <vector <int> > comb(n + 1, vector <int> (k + 1, 0));
    for (int i = 0; i < n; i++) {
        dp[i][0] = 1.0;
        comb[i][0] = 1;
    }

    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] * p[i - 1];
            comb[i][j] = comb[i - 1][j] + comb[i - 1][j - 1];
        }
    }
    out << fixed << setprecision(10) << dp[n][k] / comb[n][k] << '\n';
    return 0;
}