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

void bkt(int poz, double crt)
{
    for(int i = 1; i <= n; i++)
    {
        if(!used[i])
        {
            used[i] = 1;
            if(poz < k)
                bkt(poz + 1, crt * p[i]);
            else
            {
                s += crt * p[i];
                cnt++;
            }
            used[i] = 0;
        }
    }
}

int main()
{
    fin >> n >> k;
    for(int i = 1; i <= n; i++)
        fin >> p[i];
    bkt(1, 1.0);
    fout << fixed << setprecision(6) << s / cnt;

    return 0;
}