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

public class Main {
    public static void main(String[] args) {
        try {
            // 標準入力
            InputStreamReader isr = new InputStreamReader(System.in);
            BufferedReader br = new BufferedReader(isr);
            String a = br.readLine();
            int aCount = 0;
            String b = br.readLine();
            int bCount = 0;
            String c = br.readLine();
            int cCount = 0;;
            String now = "a";
            while (true) {
                if ("a".equals(now)) {
                    if (aCount >= a.length()) {
                        System.out.println("A");
                        break;
                    }
                    now = a.substring(aCount, aCount + 1);
                    aCount++;
                } else if ("b".equals(now)) {
                    if (bCount >= b.length()) {
                        System.out.println("B");
                        break;
                    }
                    now = b.substring(bCount, bCount + 1);
                    bCount++;
                } else if ("c".equals(now)) {
                    if (cCount >= c.length()) {
                        System.out.println("C");
                        break;
                    }
                    now = c.substring(cCount, cCount + 1);
                    cCount++;
                }
            }
       } catch (IOException e) {
            e.printStackTrace();
       }
    }
}

/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