import java.util.*;

public class FindExtraChar {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String str1 = sc.nextLine();
        String str2 = sc.nextLine();

        int l1 = str1.length();
        int l2 = str2.length();

        if (Math.abs(l1 - l2) != 1) {
            System.out.println("Invalid input");
            return;
        }

        // int sum1 = 0, sum2 = 0;
        // for (int i = 0; i < l1; i++) sum1 += str1.charAt(i);
        // for (int i = 0; i < l2; i++) sum2 += str2.charAt(i);

        char extra = (char) Math.abs(str2-str1);
        System.out.println(extra);

        sc.close();
    }
}