#include<bits/stdc++.h>
using namespace std;
int main(){
	int s;
	cin >> s;
	char a,b;
	string s1,s2;
	cin >> s1 >> s2;
	for(int i=0;i<s;i++){
		a = s1[i];
		b = s2[i];
		if(a==b){
			a += 1;
			cout << a;
			a -= 1;
		}
		if(a>b){
			cout << b;
		}
		if(a<b){
			cout << "-1";
			return 0;
		}
	}
	return 0;
}