#include <bits/stdc++.h>
using namespace std;
#define LL long long
const int N=1e6+10;
int n;
string s1,s2,ans;
int main()
{
	ios::sync_with_stdio(false);
	cin.tie(0);cout.tie(0);
	cin>>n>>s1>>s2;
	for(int i=0;i<=s1.size();i++)
	{
		if(s1[i]<s2[i])
		{
			cout<<-1;
			return 0;
		}
		if(s1[i]!=s2[i])  ans+=s2[i];
		else ans+=s1[i]+1;
	}
	cout<<ans<<'\n';
	return 0;
}

36分有问题吗？