C++, giải cơ bản nhất ạ

#include <bits/stdc++.h>
using namespace std;
#define int long long
signed main(){
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	string s;
	cin>>s;
	if(s.size()%2==1)cout<<"-1";
	else{
		int ans1=0,ans2=0;
		for(int i=0;i<s.size()/2;i++){
			ans1=ans1*10+(s[i]-'0');
		}
		for(int i=s.size()/2;i<s.size();i++){
			ans2=ans2*10+(s[i]-'0');
		}
		if(ans1==ans2)cout<<ans1;
		else cout<<-1;
	}
}