有这样一道智力题：“某商店规定：三个空汽水瓶可以换一瓶汽水。

#include <iostream>

int main() {
    int num;
    
    while(std::cin>>num) {
        int mod, left, sum=0;
        
        left = num % 3;
        while(num/3) {
            mod = num / 3;
            left = num % 3;
            sum += mod;
            
            num = mod + left;
            left = num % 3;
        }
        if(left == 1) {
            sum += 0;
        }
        if(left == 2) {
            sum += 1;
        }
        std::cout<<sum<<std::endl;
    }
    return 0;
}