#include<fstream.h>
#include<iomanip.h>
long double v[26],sum,p;
int main()
{ifstream f("dezastru.in");
ofstream g("dezastru.out");
int n,k,i,j,l,ok;
f>>n>>k;
for(i=1;i<=n;i++) f>>v[i];
f.close();
if(k<=2) ok=2;
 else ok=1;
for(i=1;i<n;i++)
 {p=v[i];
  for(j=i+1,l=2;l<=k;j++,l++)
    p=p*v[j];
  sum=sum+p;
   for(j=i+2;j<=n;j++)
   {p=p/v[j-1]*v[j+k-ok];
    sum=sum+p;
   }
 }
cout<<sum<<'\n';
sum=sum/(((n-1)*n)/2);
g<<setprecision(6)<<sum<<'\n';
g.close();
return 0;
}