package yukicoder;

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.math.BigInteger;

public class Main {
	public static void main(String[] args) {
		BufferedReader read = new BufferedReader(new InputStreamReader(System.in));
		try {
			String[] firBox = read.readLine().split(" ");
			String[] secBox = read.readLine().split(" ");

			BigInteger first = new BigInteger(firBox[1]);
			BigInteger second = new BigInteger(secBox[1]);

			if (first.equals(second)) {
				System.out.println(-1);
			} else if (first.max(second).equals(first)) {
				System.out.println(firBox[0]);
			} else {
				System.out.println(secBox[0]);
			}

		} catch (Exception e) {
			e.printStackTrace();
		}
	}
}