C :

#include<stdio.h>
int main()
{
	int sum,a,n;
	while (~scanf("%d",&n)&&n!=0)
	{
		sum=0; a=n/3;
		while (a!=0)
		{
		sum+=(n/3);
		a=n/3;
		n=n-3*a;
		n=n+a;
	    }
	    if (n==2) sum++;
	    printf("%d\n",sum);
	}
}

C++ :

#include<iostream>
#include<cmath>
#include<cstdio>
#include<cstdlib>
#include<cstring>
using namespace std;
int main()
{
 int i,m,n=1,j,k=0;
 while (true)
 {
  cin>>n;
  if (n==0) break;
  k=0;
  if (n<3) cout<<"0";
   else
   {
   k=0;
   while (true)
    {
	 j=n/3;
	 k+=j;
	 n=n%3;
	 n+=j;
	 if (n==2)
	  k+=1;
     if (n<3)
	  break;
    }
   cout<<k<<endl;
   }
 }
 return 0;
}