import java.util.Scanner;

public class Main {
	public static void main(String[] args){
		
		Scanner sc = new Scanner(System.in);
		char [] a = sc.next().toCharArray();
		char [] b = sc.next().toCharArray();
		char [] c = sc.next().toCharArray();
		char turn = 'a';
		int an = 0;
		int bn = 0;
		int cn = 0;
		
		
		roop : while(true){
		switch(turn){
		case 'a' : if(a.length == an){System.out.println("A"); break roop;} 
		           turn = a[an]; an += 1; break;
		case 'b' : if(b.length == bn){System.out.println("B"); break roop;} 
				   turn = b[bn]; bn += 1; break;
		case 'c' : if(c.length == cn){System.out.println("C"); break roop;} 
				   turn = c[cn]; cn += 1; break;
		default :}}
	}}
/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