import java.util.Scanner;

public class Test {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		String p1 = sc.next(), x = sc.next();
		String p2 = sc.next(), y = sc.next();
		if(p1.compareTo(p2) > 0){
			System.out.println(p1);
		} else if(p1.compareTo(p2) < 0){
			System.out.println(p2);
		} else {
			System.out.println(-1);
		}
	}
}