#include<fstream>
using namespace std;
int st[100],n,fact=1,k;
double cost_total=0,cost[100];
ifstream fin("dezastru.in");
ofstream fout("dezastru.out");
void afisare (int p)
  { int i;
  double cost_atac=1;
     for(i=1;i<=k;i++)
        cost_atac=cost_atac*cost[st[i]];

     cost_total=cost_total+cost_atac;
        }
int valid (int p)
  {  int i;
     for(i=1;i<p;i++)
        if(st[i]==st[p]) return 0;
        return 1;}
void back(int p)
{ int pval; //valorile pentru nivelul p;
  for(pval=st[p-1]+1;pval<=n;pval++)
    {st[p]=pval;
     if(valid(p))
        if(p==k) afisare(p);
        else back(p+1);}}

int main()
{    int i;
     fin>>n;
     fin>>k;
     for(i=1;i<=n;i++)
        fin>>cost[i];
     for(i=1;i<=n;i++)
         fact=fact*i;
     back(1);

     fout<<(cost_total*2)/fact;
     fin.close();
     fout.close();
return 0;
}