L1-025 Positive integer A+B (simple method)

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#include<iostream>
#include<string>
#include<bits/stdc++.h>
using namespace std;
int cha(char *str)
{
    
    
    int sum=0;
    for(int i=0; str[i]!='\0'; i++)
    {
    
    
        if(str[i]>='0'&&str[i]<='9')
        {
    
    

            sum=sum*10+str[i]-'0';
        }
        else
            return -1;
    }
    if(sum>=1&&sum<=1000)
        return sum;
    else
        return -1;
}
int main()
{
    
    
    char a[10001],b[10001];
    cin>>a;
    cin>>b;
    int n=cha(a);
    int m=cha(b);
    if(n==-1)
    printf("? + ");
    else
    printf("%d + ",n);
    if(m==-1)
    printf("? = ");
    else
    printf("%d = ",m);
    if(n==-1||m==-1)
        cout<<"?";
    else
        printf("%d",n+m);
    return 0;
}