#include <bits/stdc++.h>

using namespace std;

ifstream fin("dezastru.in");
ofstream fout("dezastru.out");

double v[26];
double sum;
int cate;
int n, k;
void btracking (int pas, double produs, int ultimul)
{
    if (pas<=k)
    {
        int d = n - (k-pas);
        for (int i=ultimul+1;i<=d;++i)
            btracking(pas+1, produs*v[i], i);
    }
    else
    {
        sum+=produs;
        ++cate;
        return;
    }
}

int main()
{
    fin>>n>>k;
    for (int i=1;i<=n;++i)
        fin>>v[i];
    btracking(1, 1.0, 0);
    fout<< fixed << setprecision(6) << sum/cate;

}