#include <fstream>
#include <iomanip>
#include <algorithm>
using namespace std;

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

const int N=30;

int n, k, p[N];
double v[N];

int main()
{
    fin>>n>>k;
    for(int i=0;i<n;++i)
        fin>>v[i], p[i]=i;

    double s=0;
    do
    {
        double pr=1;
        for(int i=0;i<k;++i)
            pr*=v[p[i]];
        s+=pr;
    } while(next_permutation(p, p+n));

    fout<<fixed<<setprecision(6)<<s/6;
    return 0;
}