import java.util.*;
public class PseudoEquivalentStrings {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
int N = scanner.nextInt();
scanner.nextLine(); // Consume the newline character
String w1 = scanner.next();
String w2 = scanner.next();
if (arePseudoEquivalent(w1, w2)) {
String result = (w1.compareTo(w2) < 0) ? w1 : w2;
System.out.println(result);
} else {
System.out.println("-1");
}
}
public static boolean arePseudoEquivalent(String w1, String w2) {
int[] count1 = new int[26];
int[] count2 = new int[26];
for (char c : w1.toCharArray()) {
count1[c - 'a']++;
}
for (char c : w2.toCharArray()) {
count2[c - 'a']++;
}
for (int i = 0; i < 26; i++) {
if (Math.abs(count1[i] - count2[i]) > 3) {
return false;
}
}
return true;
}
}