#include <fstream>
#include <iomanip>
#define VAL 35

using namespace std;

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

int N, K, i;
int x[VAL];
double v[VAL], sum;
double P, Xfact, Nfact, Kfact;

void back(int p)
{
    int i;
    if (p==K)
    {
        P=1;
        for (i=1; i<=K; i++)
          P*=v[x[i]];
        sum+=P;
    }
    else
    {
        for (i=x[p]+1; i<=N; i++)
        {
            x[p+1]=i;
            back(p+1);
        }
    }
}

int main()
{
    fin >> N >> K;
    Xfact=Nfact=Kfact=1;
    for (i=1; i<=N; i++)
    {
        fin >> v[i];
        Nfact*=i;
        if (i<=K)
        {
            Kfact*=i;
            x[i]=i;
        }
        else
          Xfact*=i-K;
    }
    back(0);
    sum*=Kfact*Xfact;
    sum/=Nfact;
    fout << fixed << setprecision(6) << sum << '\n';
    fin.close();
    fout.close();
    return 0;
}