请问一下代码哪里出错了

#include<stdio.h>
#include<string.h>

int main(){
    char num1[100];
    char num2[100];
    int i = 0;
    int n = 0;
    int m = 0;
    int j = 0;

    printf("输入两个字符串: ");
    fgets(num1, sizeof(num1), stdin);
    fgets(num2, sizeof(num2), stdin);

    while(num1[i] != '\0'){
        n = n * 10 + num1[i] - '0';
        i++;
    }

    while(num2[j] != '\0'){
        m = m * 10 + num2[j] - '0';
        j++;
    }

    int sum = m + n;
    char sum_str[10];
    sprintf(sum_str, "%d", sum);
    printf("%s", sum_str);
    return 0;
}