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

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

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

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

}