#include<iostream>
#include<string>
#include<cmath>
using namespace std;
string s1,s2,g;
int n;
string f(string _s1,string _s2)
{
	if(_s1.length()!=_s2.length())
	{
		return "-1"; 
	}
	for(int i=0;i<n;i++)
	{
		g+=char(min(int(_s1[i]),int(_s2[i])));
	}
	return g;
}
int main()
{
	cin>>n>>s1>>s2;
	cout<<f(s1,s2);
	return 0;
}