#include <bits/stdc++.h>
 
using namespace std;
 
ifstream in("dezastru.in");
ofstream out("dezastru.out");

int n, k, solutions;
double ans;
vector< double > vals(30), sol(30);
vector< bool > viz(30, false);

void bkt(int poz) {
    if(poz == k) {
        double temp = 1.0;
        for(int i = 0; i < k; ++i) {
            temp *= sol[i];
        }

        ans += temp;
        ++solutions;

        return;
    } else {
        for(int i = 0; i < n; ++i) {
            if(!viz[i]) {
                sol[poz] = vals[i];

                viz[i] = true;
                bkt(poz + 1);
                viz[i] = false;
            }
        }
    }
}
 
int main() {
    ios::sync_with_stdio(false); in.tie(0); out.tie(0);
    
    in >> n >> k;
    for(int i = 0; i < n; ++i) {
        in >> vals[i];
    }

    bkt(0);

    ans /= double(solutions);

    out << fixed << setprecision(6) << ans << "\n";
 
    in.close(); out.close();
 
    return 0;
}