#include <bits/stdc++.h>
using namespace std;

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

int n, k, ind[35], divis;
double v[35], sum;

void bkt(int index, double product) {
    for(int i = ind[index - 1] + 1; i <= n; i++) {
        ind[index] = i;
        if(index == k) {
            sum += product * v[ind[index]];
            divis++;
        } else {
            bkt(index + 1, product * v[ind[index]]);
        }
    }
}

int main() {

    in >> n >> k;
    for(int i = 1; i <= n; i++) {
        in >> v[i];
    }

    bkt(1, 1);
    out << (double)sum / divis << endl;
    return 0;
}