import java.util.ArrayDeque;
import java.util.Scanner;

public class Main{
	
	static final Scanner s=new Scanner(System.in);
	
	public static void main(String[] __){
		D[] deques = new D[3];
		for(int i=0;i<3;i++) {
			deques[i]=new D();
			for(char c:s.next().toCharArray())
				deques[i].add(c);
		}
		char next='a';
		while(!deques[next-'a'].isEmpty()) {
			next=deques[next-'a'].poll();
		}
		System.out.println(Character.toUpperCase(next));
	}
	static class D extends ArrayDeque<Character>{}
}
/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