求助！为什么无法通过/(ㄒoㄒ)/~~！测试样例代入是对的呀

#include <stdio.h>
int main() {
    int y,sum,n;
    scanf("%d",&n);
    while(n!=0){
        y=n%10;
        sum=sum*10+y;
        n=n/10;
    }
    printf("%d",sum);
    return 0;
}