#include <bits/stdc++.h>

using namespace std;

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

int v[26];
int n, m, l;
double a[26], b[26];
long double sum;

void back(int k){

    for(int i = v[k - 1] + 1;i <= n;i++){
        v[k] = i, b[k] = b[k - 1] * v[i];
        if(k == m) 
            sum += b[k], l++;
        else back(k + 1);
    }
}

int main(){
    f >> n >> m;
    for(int i = 1;i <= n;i++)
        f >> a[i];
    b[0] = 1;
    back(1);
    g << fixed << setprecision(6) << sum / l;
}