#include <stdio.h>
int main()
{
int n;
scanf("%d",&n);
int sec=0;
int tl=0;//tili
int count=0;
while(n>0)
{
//printf("n:%d sec:%d tl:%d\n",n,sec,tl);
if(tl>3)
{
tl-=3;
n-=3;
continue;
}
if(n>=2)
{
tl+=1;
sec+=1;
n-=2;
continue;
}
if(n<2)
{
tl+=1;
sec+=1;
n-=1;
continue;
}
}
printf("%d\n",sec);
return 0;
}