#include <iostream>
#include <fstream>

using namespace std;

int n, k;
double a[100];
int vis[100], v[100];
double total_sum;
double total_ans;

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

bool valid(int x) {
    if (vis[x] == 1) return false;
    return true;
}

void calculare() {
    double current_val = 1;
    for (int i = 1; i <= k; ++i) {
        //cout << v[i] << " ";
        current_val *= a[v[i]];
    }
    total_sum += current_val;
    //cout << "\n";
    total_ans ++;
}

void bk(int x) {

    for (int i = 1; i <= n; ++i) {
        v[x] = i;
        if (valid(i)) {
            vis[i] = 1;
            if (x == k) {
                calculare();
            } else bk(x + 1);
        }
        vis[i] = 0;
    }

}

int main() {

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

    bk(1);

    g << (total_sum / total_ans);

    return 0;
}