#include <bits/stdc++.h>
using namespace std;
ifstream fin("dezastru.in");
ofstream fout("dezastru.out");
int n, k, a[26], cnt = 0;
double p[26], s = 0;

int main()
{
    fin >> n >> k;
    for(int i = 1; i <= n; i++)
        fin >> p[i];
    iota(a + 1, a + n + 1, 1);
    do
    {
        cnt++;
        double crt = 1;
        for(int i = 1; i <= k; i++)
            crt *= p[a[i]];
        s += crt;
    }while(next_permutation(a + 1, a + n + 1));
    fout << fixed << setprecision(6) << s / cnt;

    return 0;
}