PAT 1001. A+B Format

#include<stdio.h>
int a, b;
int sum;
int main(){
	while (scanf("%d %d", &a, &b)!=EOF){
		sum = a + b;
		if (sum < 1000&&sum>-1000)printf("%d\n", sum);
		else{
			int sum1 = sum % 1000;
			int sum2 = sum / 1000;
			int sum3 = 0;
			if (sum2 != 0){
				sum3 = sum2 / 1000;
			}
			sum2 = sum2 % 1000;
			if (sum3 != 0){
				printf("%d,%03d,%03d\n", sum3 % 1000, sum2 < 0 ? -sum2 : sum2, sum1 < 0 ? -sum1 : sum1);
			}
			else
			printf("%d,%03d\n", sum2, sum1<0?-sum1:sum1);
			
		}
		
	}
}