import java.io.*;
import java.math.*;
import java.util.*;
public class Main {  
 
	public static void main(String[] args) {  
		FastScanner I = new FastScanner(); //Input
		OutPut O = new OutPut(); //Output
		String a = I.next().toUpperCase();
		String b = I.next().toUpperCase();
		String c = I.next().toUpperCase();
		Queue<Character> A = new LinkedList<Character>();
		Queue<Character> B = new LinkedList<Character>();
		Queue<Character> C = new LinkedList<Character>();
		char cur = 'A';
		String ans = "";
		for (int i = 0; i < a.length(); i++) A.add(a.charAt(i));
		for (int i = 0; i < b.length(); i++) B.add(b.charAt(i));
		for (int i = 0; i < c.length(); i++) C.add(c.charAt(i));
		while (true) {
			if (cur=='A') {
				if (A.size()==0) {
					ans="A";
					break;
				}
				cur=A.poll();
			}else if (cur=='B') {
				if (B.size()==0) {
					ans="B";
					break;
				}
				cur=B.poll();
			}else {
				if (C.size()==0) {
					ans="C";
					break;
				}
				cur=C.poll();
			}
			//O.pln(cur);
		}
		O.pln(ans);
	}
	public static long ceil(long num, long den) {long ans = num/den; if (num%den!=0) 
	ans++; return ans;}
	public static long GCD(long a, long b) {
		if (a==0||b==0) return Math.max(a,b);
		return GCD(Math.min(a, b),Math.max(a, b)%Math.min(a, b));
	}
	public static long FastExp(long base, long exp, long mod) {
		long ans=1;
		while (exp>0) {
			if (exp%2==1) ans*=base;
			exp/=2;
			base*=base;
			base%=mod;
			ans%=mod;
		}
		return ans;
	}
	public static long ModInv(long num,long mod) {return FastExp(num,mod-2,mod);}
	public static int pop(long x) { //Returns number of bits within a number
		int cnt = 0;
		while (x>0) {
			if (x%2==1) cnt++;
			x/=2;
		}
		return cnt;
	}
	static class FastScanner {
		BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
		StringTokenizer st=new StringTokenizer("");
		String next() {
			while (!st.hasMoreTokens())
				try {
					st=new StringTokenizer(br.readLine());
				} catch (IOException e) {
					e.printStackTrace();
				}
			return st.nextToken();
		}
		int nextInt() {return Integer.parseInt(next());}
		long nextLong() {return Long.parseLong(next());};
	}
	static class OutPut{
		PrintWriter w = new PrintWriter(System.out);
		void pln(double x) {w.println(x);w.flush();}
		void pln(boolean x) {w.println(x);w.flush();}
		void pln(int x) {w.println(x);w.flush();}
		void pln(long x) {w.println(x);w.flush();}
		void pln(String x) {w.println(x);w.flush();}
		void pln(char x) {w.println(x);w.flush();}
		void pln(StringBuilder x) {w.println(x);w.flush();}
		void pln(BigInteger x) {w.println(x);w.flush();}
		void p(int x) {w.print(x);w.flush();}
		void p(long x) {w.print(x);w.flush();}
		void p(String x) {w.print(x);w.flush();}
		void p(char x) {w.print(x);w.flush();}
		void p(StringBuilder x) {w.print(x);w.flush();}
		void p(BigInteger x) {w.print(x);w.flush();}
		void p(double x) {w.print(x);w.flush();}
		void p(boolean x) {w.print(x);w.flush();}
	}
}
/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