#include <stdio.h>

int main()
{   
	int n = 0;
	scanf("%lld", &n);
	getchar();
	char x[120] = { 0 };
	char y[120] = { 0 };
	/*scanf("%s", &x);
	getchar();
	scanf("%s", &y);*/
	gets(x);
	gets(y);
	
	for (int i = 0; i < n; i++)
	{
		if (y[i]>x[i])
		{
			printf("-1");
			return 0;
		}
			
	}
		printf("%s", y);
		
		return 0;
	}