import java.util.Scanner;

public class Main {
  public static void main(String[] args) {
    final Scanner scanner = new Scanner(System.in);

    final String[] s = new String[3];
    s[0] = scanner.nextLine();
    s[1] = scanner.nextLine();
    s[2] = scanner.nextLine();

    final int[] idx = new int[3];
    int nextTurn = 0;
    while (idx[nextTurn] < s[nextTurn].length()) {
      final char c = s[nextTurn].charAt(idx[nextTurn]);
      idx[nextTurn]++;

      nextTurn = c - 'a';
    }

    System.out.println(Character.toUpperCase((char) (nextTurn + 'a')));
  }
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz