#include <fstream>
#include <iomanip>
using namespace std;
ifstream f ("dezastru.in");
ofstream g ("dezastru.out");

int n, m, t, aux[31];
double v[31], s, scrt = 1;

void back(int k) {
    if(k == m + 1) {
        s += scrt;
        t++;
    } else {
        for(int i = aux[k - 1] + 1; i <= k + n - m; i++) {
            aux[k] = i; scrt *= v[i];
            back(k + 1);
            scrt /= v[i];
        }
    }
} 

int main () {
    f >> n >> m;
    for(int i = 1; i <= n; i++)
        f >> v[i];
    back(1);
    g << fixed << setprecision(6) << s / t;
    
    return 0;
}