#include<stdio.h>
#include<string.h>

int main(){
	int n,i,flag = 0;
	scanf("%d",&n);
	char x[n],y[n],z[n];
	scanf("%s",x);
	scanf("%s",y);
	for(i = 0; i < n; i++)
	{
		if(x[i] < y[i])
		{
			printf("-1");
			flag = 1;
			break;
		}
		else if(x[i] >= y[i])
		{
			z[i] = y[i];
		}
	}
	if(flag == 0)
	{
		printf("%s\n",z);
	}
	
	return 0;
}