100%AC

#include<bits/stdc++.h>
using namespace std;
string s1,s2;
long long a[100000],b[100000],c[100000],z;
int main(){
    cin>>z;
    cin>>s1;
    for(int i=0;i<s1.size();i++){
        if(z>10&&s1[i]>='A'){
            a[s1.size()-i]=s1[i]-'A'+10;
        }
        else a[s1.size()-i]=s1[i]-'0';
    }
    cin>>s2;
    for(int i=0;i<s2.size();i++){
        if(z>10&&s2[i]>='A'){
            b[s2.size()-i]=s2[i]-'A'+10;
        }
        else b[s2.size()-i]=s2[i]-'0';
    }
    long long len=max(s2.size(),s1.size())-1;
    int x=0,y=0;
    while(x<=s1.size()||x<=s2.size()){
        x++;
        c[x]+=a[x]+b[x]+y;
        y=c[x]/z;
        c[x]%=z;
    }
    while(c[x]==0&&x>1)x--;
    for(int i=x;i>=1;i--){
        if(c[i]<10)cout<<c[i];
        else cout<<char(c[i]+'A'-10);
    }
    return 0;
}