#include <fstream>
#include <iomanip>
using namespace std;
ifstream fin("dezastru.in");
ofstream fout("dezastru.out");
int n, m, sd;
int v[30];
double c[30], s;
void back(int k) {
    if (k > m) {
        float d = 1;
        for (int i = 1; i < k; ++i) {
            d *= c[v[i]];
        }
        sd++;
        s += d;
    }
    else {
        for (int i = v[k - 1] + 1; i <= n; ++i) {
            v[k] = i;
            back(k + 1);
            v[k] = 0;
        }
    }
}
int main() {
    fin >> n >> m;
    for (int i = 1; i <= n; ++i) {
        fin >> c[i];
    }
    back(1);
    fout << setprecision(6);
    fout << fixed;
    fout << s / sd;
    fout.close();
    return 0;
}