rt，感觉是测评问题

#include<bits/stdc++.h>
using namespace std;
string f(string s1,string s2){
	string r;
	for (int i=1;i<=s1.length();i++){
		r[i]=min(s1[i],s2[i]);
	}
	return r;
}

int main(){
	int n;
	string x,y;
	cin>>n;
	cin>>x;
	cin>>y;
	for (int i=1;i<=n;i++){
		if (x[i]<y[i]){
			cout<<-1<<endl;
			return 0;
		}
	}
	cout<<y;
	return 0;
}