import java.io.*;
import java.util.*;
import java.math.*;

class Main201 {
	
	public static void out (Object out) {
		System.out.println(out);
	}
	
	public static int comp (String s1, String s2) {
		int len1 = s1.length();
		int len2 = s2.length();
		
		if (len1 > len2) {
			return 1;
		} else if (len1 < len2) {
			return -1;
		}
		
		for (int i = 0; i < len1; i++) {
			int num1 = s1.charAt(i) - '0';
			int num2 = s2.charAt(i) - '0';
			if (num1 > num2) {
				return 1;
			} else if (num1 < num2) {
				return -1;
			}
		}
		
		return 0;
	}
	
	public static void main (String[] args) throws IOException {
		BufferedReader br = 
			new BufferedReader(new InputStreamReader(System.in));
		
		String[] line1 = br.readLine().split(" ");
		String[] line2 = br.readLine().split(" ");
		
		int c = comp(line1[1], line2[1]);
		
		System.out.println(c == 1 ? line1[0] : c == -1 ? line2[0] : -1);
		
	}
}