import java.util.*;

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

        // Length check
        if (t.length() - s.length() != 1) {
            System.out.println("Invalid input");
            return;
        }

        // Frequency array for ASCII (0–255)
        int[] freq = new int[256];

        // Count for s
        for (char c : s.toCharArray()) {
            freq[c]--;
        }

        // Count for t
        for (char c : t.toCharArray()) {
            freq[c]++;
        }

        // Find the character with freq = 1
        for (int i = 0; i < 256; i++) {
            if (freq[i] == 1) {
                System.out.println((char) i);
                return;
            }
        }
    }
}