#include<stdio.h>
using namespace std;
const int N = 30;
double v[N], nrsol, s;
bool viz[N];
int n, k, sol[N], ct, ult;
void bkt ()
{
    if (ct == k)
    {
        nrsol++;
        double p = 1;
        int i;
        for (i = 1; i <= ct; i++)
            p *= v[sol[i]];
        s += p;
        return;
    }

    int i;
    for (i = ult; i <= n; i++)
    {
        if (viz[i] == false)
        {
            sol[++ct] = i;
            viz[i] = true;
            ult = i + 1;
            bkt();
            viz[i] = false;
            ct--;
        }
    }
}
int main ()
{
    FILE *in, *out;
    in = fopen ("dezastru.in", "r");
    out = fopen ("dezastru.out", "w");
    fscanf (in, "%d%d", &n, &k);
    int i;
    for (i = 1; i <= n; i++)
        fscanf (in, "%lf", &v[i]);
    ult = 1;
    bkt();
    fprintf (out, "%lf", s / nrsol);
    return 0;
}