#include <bits/stdc++.h>
using namespace std;
int v[101],uz[101],n,k,loc;
double p,x[101],s;
void Back(int top)
{    int i;
    if(top>=n+1)
               {p=1;
                   for(i=1;i<=k;i++)p=p*x[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()
{
    int i;
cin>>n>>k;
for (i=1;i<=n;i++) cin>>x[i];
Back(1);
s=s/loc;
cout<<setprecision(7)<<s;}