Help me in solving STRFIRSTCHAR problem

help
,
1

My issue

how the answer is 1 for:
1
2 3
aa
aba

shouldn’t it be 2 ?

My code

#include<iostream>
#define ll long long
using namespace std;

void fun(string &s1, string &s2){
    int ans=0;
    int a=s1.size(), b=s2.size();
    bool flag=false;
    int i=0, j=0;
    while(i<a && j<b){
        if(s1[i] != s2[j]){
            if(i>0) ans=2;
            else{
                flag=true;
                ans=-1;
            } 
            break;
        }
        ++i;++j;
    }
    if(a!=b && ans==0 && flag==false) ans=1;
    cout<<ans<<"\n";
}

int main(){
    int t;
    cin>>t;
    for(int q=0;q<t;++q){
        int a,b;
        cin>>a>>b;
        string s1,s2;
        cin>>s1;
        cin>>s2;
        fun(s1,s2);
    }
    return 0;
}

Problem Link: Replace With First Practice Coding Problem