import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        String[] array = new String[3];
        int[] cnt = new int[3]; 
        int i,j=0;
        for(i=0;i<3;i++) array[i] = sc.next();
        while(true){
            if(cnt[j]==array[j].length()){
                System.out.println((char)(j+'A'));
                break;
            }
            j = array[j].charAt(cnt[j]++)-'a';
        }
    }
}

/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