import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String [] S = {sc.next(),sc.next(),sc.next()};
        int [] k = {0,0,0};
        int t=0;
        while(true){
            k[t]++;
            for(int i=0; i<3; i++){
                if(k[i]>S[i].length()){
                    char c = (char)('A'+i);
                    String ans = String.valueOf(c);
                    System.out.println(ans);
                    return;
                }
            }
            t = S[t].charAt(k[t]-1)-'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