#include <fstream>
#include <cmath>
using namespace std;
int i,j,n,k,t[100],p;
double d[100],s;
   ifstream fin("dezastru.in");
    ofstream fout("dezastru.out");

void bk(int l)
{
    if (l==k)
    {
        double j=1;
        for (int i=1;i<=k;i++)
            j*=d[t[i]];
            s=s+j/p*2;
            return;
    }
    for (int i=t[l]+1;i<=n;i++)
    {
        t[l+1]=i;
        bk(l+1);
    }
}
int main()
{

    fin>>n>>k;
    for (i=1;i<=n;i++) fin>>d[i];
    p=1;
    for (i=n-k+1;i<=n;i++) p*=i;
    bk(0);
    fout<<round(s*1000000)/1000000
    ;
    return 0;
}