# include <stdio.h>

void fun(char * s1, int n1, char * s2, int n2)
{
	int i, k=1;
	for(i=0; i<n1; i++)
		if((s1[i]<s2[i]))
		{
			printf("%d\n", -1);
			k=0;
		}
	if(k!=0)
		for(i=0; i<n1; i++)
			printf("%c",s2[i]);
	printf("\n");
}

int main(void)
{
	int n;
	char s1[100], s2[100];

	scanf("%d", &n);
	scanf("%s", s1);
	scanf("%s", s2);

	fun(s1, n, s2, n);

	return 0;
}