#include <iostream>
#include<fstream>
using namespace std;
int uz[101],n,v[101],k,loc;
float a[101],s,p;
void Back(int top)
{    int i,x;
    if(top>=n+1)
               {p=1;
                for(i=1;i<=k;i++)p=p*a[v[i]];
                 s=s+p;
                 loc++;}

    else for(i=1;i<=n;i++)

    if(uz[i]==0){uz[i]=1;
                  v[top]=i;
                  Back(top+1);
                  uz[i]=0;}

}
int main()
{ifstream fin("dezastru.in");
ofstream fout("dezastru.out");
fin>>n>>k;
for(int i=1;i<=n;i++)fin>>a[i];
Back(1);
fout<<s/loc;}