import java.util.Scanner;

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

		String[] s = {sc.nextLine(), sc.nextLine(), sc.nextLine()};

		int turn = 'a';
		while (true) {
			if (s[turn - 'a'].length() == 0) {
				break;
			}
			char c = s[turn - 'a'].charAt(0);
			s[turn - 'a'] = s[turn - 'a'].substring(1);
			turn = c;
		}
		System.out.println(turn == 'a' ? "A" : turn == 'b' ? "B" : "C");
	}
}


/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