import java.util.*;
public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		String a = sc.next();
		String b = sc.next();
		String c = sc.next();
		char next = 'a';
		while(true) {
			if(next == 'a') {
				if(a.length()==0) {
					System.out.println("A");
					break;
				}
				next = a.charAt(0);
				a = a.substring(1);
			}else if(next == 'b') {
				if(b.length()==0) {
					System.out.println("B");
					break;
				}
				next = b.charAt(0);
				b = b.substring(1);
			}else {
				if(c.length()==0) {
					System.out.println("C");
					break;
				}
				next = c.charAt(0);
				c = c.substring(1);
			}
	//		System.out.println(next);
		}
	}

}
/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