#include <fstream>
#include <iomanip>

using namespace std;

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

int n, k, i, j;
double a[26], s;
int b[26];
unsigned long long nf;

void back(int t)
{
    int i;
    if (t == k+1)
    {
        double p = 1;
        for (i = 1; i <= k; i++)
            p *= a[b[i]];
        s += (2*p);
    }
    else
    {
        for (i = b[t-1]+1; i <= n; i++)
        {
            b[t] = i;
            back(t+1);
        }
    }
}

int main()
{
    f >> n >> k;
    for (i = 1, nf = 1; i <= n; i++)
        nf *= i;
    for (i = 1; i <= n; i++)
        f >> a[i];
    back(1);
    g <<fixed<<setprecision(6)<<(double) s/nf;
    return 0;
}