#include <fstream>

using namespace std;

ifstream f("dezastru.in");
ofstream g("dezastru.out");

int n, p, x[30], cnt;
double v[30], prod[30], sol;

void backt(int k)
{
    for(int i = x[k - 1] + 1;  i <= n;  i++)
    {
        x[k] = i, prod[k] = prod[k - 1] * v[i];
        if(k >= p)
            sol += prod[k] , cnt++;
        else
            backt(k + 1);
    }
}

int main()
{
    f >> n >> p;
    for(int i = 1; i <= n; i++)
        f >> v[i];
    prod[0] = 1;
    backt(1);

    g << sol / cnt;
    return 0;

}