#include <fstream>
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
int n,m,uz[26],nr,i;
double a[26],S,SOL,sum=1;
void gen(int k,double sum)
{
    int i;
  if(k>m)
    {
        S=S+sum;
        ++nr;
    }
    else
    for(i=uz[k-1]+1;i<=n-m+k;++i)
    {
        uz[k]=i;
        gen(k+1,sum*a[i]);
    }
}
int main()
{
    f>>n>>m;
    for(i=1;i<=n;++i)
        f>>a[i];
    gen(1,1.0);
    SOL=S/nr;
    g<<SOL;
    return 0;
}