import java.io.IOException;
import java.util.Scanner;


public class Main {

	public static void main(String[] args) throws IOException {
		Scanner scanner = new Scanner(System.in);
		String aString = scanner.nextLine();
		String bString = scanner.nextLine();
		String cString = scanner.nextLine();

		int a=0;
		int b=0;
		int c=0;
		char start = 'a';
		while (true) {
			if(start=='a'){

				if(a==aString.length()){
					System.out.println("A");
					break;
				}
				start = aString.charAt(a);
				a++;
			}else if (start=='b') {

				if(b==bString.length()){
					System.out.println("B");
					break;
				}
				start = bString.charAt(b);
				b++;
			}else {

				if(c==cString.length()){
					System.out.println("C");
					break;
				}
				start = cString.charAt(c);
				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