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

ifstream in("dezastru.in");
ofstream out("dezastru.out");
 
double sol = 0, v[30];
int n, m, nrf, st[30];
 
int backt(int k, int s, double sum)
{
    if (k > m)
    {
        sol += sum;
        nrf++;
        return 0;
    }
    for (int i = s + 1;i <= n - m + k; ++i)
    {
        backt(k + 1, i, sum*v[i]);
    }
}
 
int main()
{
    in >> n >> m;
    for (int i = 1;i <= n;i++)
        in >> v[i];
    backt(1, 0, 1);
    out << setprecision(6) << fixed << (double) sol / nrf;
    return 0;
}