import java.util.*;

public class Main{
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String saa = sc.next(), sbb = sc.next(), scc = sc.next();
		int na = saa.length(), nb = sbb.length(), nc = scc.length();
		int a = 0, b = 0, c = 0, turn = 0;
		while(true){
			if(turn == 0){
				if(a == na){
					System.out.println('A');
					break;
				}
				turn = saa.charAt(a) - 'a';
				a++;
			}else if(turn == 1){
				if(b == nb){
					System.out.println('B');
					break;
				}
				turn = sbb.charAt(b) - 'a';
				b++;
			}else{
				if(c == nc){
					System.out.println('C');
					break;
				}
				turn = scc.charAt(c) - 'a';
				c++;
			}
		}
		sc.close();
	}
}

/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