#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=1;pval<=n;pval++)
{st[p]=pval;
if(valid(p))
if(p==n) 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/fact;
fin.close();
fout.close();
return 0;
}