#include <fstream>
#include <algorithm>
#include <iomanip>
using namespace std;

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

float v[30],nr,rez;
int a[30],i,n,k,perm=0;

int main ()
{
    f>>n>>k;
    for (i=1; i<=n; i++)
        f>>v[i];
for (i=1; i<=n; i++)
    a[i]=i;
do {
    perm++;
    nr=1;
for (i=1; i<=k; i++)
nr*=v[a[i]];

rez+=nr;

}
while (next_permutation(a+1,a+n+1));

g<<setprecision(6)<<rez/perm;

return 0;
}