#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
double sol = 0, v[30];
int n, m, a[30], nFact = 0, i;

void citeste()
{
    f >> n >> m;
    for( i = 1 ; i <= n ; i++ )
        f >> a[i];
}

void bkt(int k, int s, double sum)
{
    if (k > m)
    {
        sol += sum;
        nFact++;
        return;
    }
    for ( i = s + 1;i <= n - m + k; ++i)
    {
         bkt(k + 1, i, sum*v[i]);
    }
}

void afisare()
{
    g << fixed << setprecision(6) << sol / nFact << "\n";
}

int main()
{
    citeste();
    bkt(1, 0, 1);
    afisare();
    return 0;
}