Zhejiang version of "C Programming Language (3rd Edition)" title set 6-7 exercises simple calculator (20 points)

#include <stdio.h>
int main()
{
    int a, b, flag;
    char op;
    flag = 0;
    scanf("%d", &a);
    while ((op = getchar()) != '=')
    { // 操作符为'='时，退出循环。
        if (op == '+')
        {
            scanf("%d", &b);
            a = a + b;
        }
        else if (op == '-')
        {
            scanf("%d", &b);
            a = a - b;
        }
        else if (op == '*')
        {
            scanf("%d", &b);
            a = a * b;
        }
        else if (op == '/')
        {
            scanf("%d", &b);
            if (b == 0)
                flag = 1; // 若除数为0，flag标记1.
            else
                a = a / b;
        }
        else
            flag = 1;
    }
    if (flag)
        printf("ERROR\n");
    else
        printf("%d", a);
    return 0;
}