#include<bits/stdc++.h>
using namespace std;
int main()
{
	int n;
	string  x;
	string  y;
	cin>>n;
	cin>>x;
	cin>>y;
	for(int i=0;i<n;i++)
	{
		if(x[i]<y[i])
		{
			cout<<"-1";
			return 0;
		}
		else if(x[i]<y[i])
		{
			cout<<x[i];
		}
		else if(x[i]=y[i])
		{
			cout<<y[i];
		}
	}
	return 0;
}