#include<stdio.h>
int main()
{
    float score;
    scanf("%f",&score);
    if(score<0){
        printf("invalid input\n");
        return 0;
    }
    if (fmod(score,1.0)!= 0.0){
        printf("Invalid Input\n");
        return 0;
    }
    int n=(int)score;
    int turns =n/3;
    if(n%3!=0){
        turns +=1;
    }
    printf("%d\n",turns);
    return 0;
}