#include<fstream.h>

int main ()

{ int n,i,k,l,o;

 ifstream f ("fact.in");
 ofstream g ("fact.out");
 f>>n;


  k=n%2;
  l=n/2;

  if(n%3==0)  o=l*9;
  if(n%3==1)  o=l*9;
  if(n%3==2)  o=l*9+1;
  g<<o;


 return 0;
}