import java.util.*;

public class Main {    
    public static void main(String[] args){        
        Scanner sc = new Scanner(System.in);                                
         
        StringBuilder sA = new StringBuilder(sc.nextLine());
        StringBuilder sB = new StringBuilder(sc.nextLine());
        StringBuilder sC = new StringBuilder(sc.nextLine());
        
        Map<Character,StringBuilder> map = new HashMap<>();
        
        map.put('a',sA);
        map.put('b',sB);
        map.put('c',sC);

        char ch = sA.charAt(0);
         
        while(true){                                  
           StringBuilder sb = map.get(ch);      
           if(sb.length() == 0){
                System.out.println(String.valueOf(ch).toUpperCase());   
                return;
           }           
           ch = sb.charAt(0);
           sb.deleteCharAt(0);                                                      
        }                        
        
    }            
}
/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