#include <fstream>
using namespace std;

ifstream fin("dezastru.in");
ofstream fout("dezastru.out");

long double p, s, a[26], kf, nkf;

int c[26], k, n;

void gen(int x)
{
    if (x == k+1)
        s = s + p * kf * nkf;
    else
    {
        int i;
        for (i = c[x-1] + 1; i<=n; i++)
        {
            p = p * a[i];
            c[x] = i;
            gen(x+1);
            p = p / a[i];
        }
    }
}

int main()
{
    int i;
    fin >> n >> k;
    for (i = 1; i<=n; i++)
        fin >> a[i];
    kf = nkf = 1;
    for (i = 1; i<=k; i++)
        kf = kf * i;
    for (i = 1; i<=n-k; i++)
        nkf = nkf * i;
    p = 1;
    gen(1);
    long double nf = 1, nr;
    for (i = 1; i<=n; i++)
        nf = nf * i;
    nr = nf;
    fout << s/nr;
    return 0;
}