// editor1
#include <stdio.h>
#include <math.h>
int main()
{
    float score;
    scanf("%f", &score);
    if(score < 0)
    {
        printf("Invalid Input");
        return 0;
    }
    if((int)score != score)
    {
        printf("Invalid Input");
        return 0;
    }
    int turns = (int)cill(score / 3.0);
    printf("%d", turns);
    return 0;
}