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

public class Main {

	public static void main(String[] args) throws Exception {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		String lineA = br.readLine();
		String lineB = br.readLine();
		String lineC = br.readLine();
		int indexA = 0;
		int indexB = 0;
		int indexC = 0;
		char ch = lineA.charAt(indexA++);
		while (true) {
			if (ch == 'a') {
				if (indexA == lineA.length()) {
					System.out.println("A");
					break;
				}
				ch = lineA.charAt(indexA++);
			}
			else if (ch == 'b') {
				if (indexB == lineB.length()) {
					System.out.println("B");
					break;
				}
				ch = lineB.charAt(indexB++);
			}
			else if (ch == 'c') {
				if (indexC == lineC.length()) {
					System.out.println("C");
					break;
				}
				ch = lineC.charAt(indexC++);
			}
		}
	}
}

/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