#include <fstream>

using namespace std;

ifstream f("dezastru.in");
ofstream g("dezastru.out");
int N, K,nr = 0;
double a[40],S = 0;

void back(int k, double jeg, int last)
{
    if (k == K + 1)
    {
        S += jeg;
        nr++;
    }
    else
    {
        for (int i = last + 1; i <= N - K + k; i++)
            back(k + 1, jeg * a[i], i);
    }
}

int main()
{

f>>N>>K;

    for (int i = 1; i <= N; i++)
        scanf("%lf", &a[i]);

    back(1, 1, 0);

g<< S / nr;


    return 0;
}