import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {
    public static void main(String[] args)throws Exception {
        BufferedReader input = new BufferedReader(new InputStreamReader(System.in));
        String [] letters = new String[3];

        for (int i = 0; i < 3; i++) {
            letters[i] = input.readLine();
        }
        int current = 0;
        int [] positions = new int[3];
        while (true){
            if (positions[current] == letters[current].length()){
                System.out.println((char)(current + 'A'));
                break;
            }
            current = letters[current].charAt(positions[current]++) - '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