#include <fstream>
using namespace std;
ifstream fin("dezastru.in");
ofstream fout("dezastru.out");

int uz[40],n,sol[40],m,t;
float atc[40],s,st;

void prelucrari();
void backtracking(int k);

int main()
{
    int i;
    fin >> n >> m;
    for (i = 1; i <= n; i++) fin >> atc[i];
    backtracking(1);
    fout << st / t;
    return 0;
}

void backtracking(int k)
{
    int i;
    if (k > n)
    {
        prelucrari();
    }
    else
    for (i = 1; i <= n; ++i)
    {
       if (uz[i] == 0)
       {
           uz[i] = 1;
           sol[k] = i;
           backtracking(k+1);
           uz[i] = 0;
       }
    }
}

void prelucrari()
{
    int k;
    int j;
    t++;
    s = 1;
    for (j = 1; j <= m; ++j) s *= atc[sol[j]];
    st = st + s;
}