using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Text;
using static System.Console;
using static System.Math;

namespace Atcoder
{
    class Program
    {
        static StreamWriter sw = new StreamWriter(OpenStandardOutput()) { AutoFlush = false };
        static void Main(string[] args)
        {
            try
            {
                SetOut(sw);
                Solve.Answer();
                Out.Flush();
            }
            catch (Exception ex)
            {
                WriteLine(ex.ToString());
            }
        }
    }

    public class Solve
    {
        public static void Answer()
        {
            var sa = ReadLine();
            var sb = ReadLine();
            var sc = ReadLine();

            var owner = 'a';
            while (true)
            {
                switch (owner)
                {
                    case 'a':
                        if (sa.Count() == 0)
                        {
                            WriteLine('A');
                            return;
                        }
                        owner = sa[0];
                        sa = sa.Remove(0, 1);
                        break;
                    case 'b':
                        if (sb.Count() == 0)
                        {
                            WriteLine('B');
                            return;
                        }
                        owner = sb[0];
                        sb = sb.Remove(0, 1);
                        break;
                    case 'c':
                        if (sc.Count() == 0)
                        {
                            WriteLine('C');
                            return;
                        }
                        owner = sc[0];
                        sc = sc.Remove(0, 1);
                        break;
                }
            }
        }

        public class IO
        {
            #region Input
            public static string ReadString => ReadLine();
            public static int ReadInt => int.Parse(ReadLine());
            public static long ReadLong => long.Parse(ReadLine());
            public static double ReadDouble => double.Parse(ReadLine());

            public static List<string> ReadSplitString => ReadLine().Split().ToList();
            public static List<int> ReadSplitInt => ReadLine().Split().Select(s => int.Parse(s)).ToList();
            public static List<long> ReadSplitLong => ReadLine().Split().Select(s => long.Parse(s)).ToList();
            public static List<double> ReadSplitdouble => ReadLine().Split().Select(s => double.Parse(s)).ToList();
            #endregion

            #region Output
            public static void OutJoinSpace<T>(IEnumerable<T> list) => WriteLine(string.Join(" ", list));
            public static void WriteLineYes_or_No(bool flag) => WriteLine(flag ? "Yes" : "No");

            #endregion
        }

        public class Mod
        {
            #region
            private readonly static long M = 1000000007;

            public static long Add(long a, long b) => (a * b) % M;
            public static long Sub(long a, long b) => (a - b) % M >= 0 ? (a - b) % M : (a - b) % M + M;
            public static long Multi(long a, long b) => (a * b) % M;
            #endregion
        }

        public static int GCD(int x, int y)
        {
            if (x < y) Swap(ref x, ref y);
            if (y == 0) return x;
            else return GCD(x % y, y);
        }

        public static void Swap(ref int x, ref int y)
        {
            int tmp = x;
            x = y;
            y = tmp;
        }

        public static long Choice2(long n)
        {
            #region
            if (n <= 1)
                return 0;
            if (n == 2)
                return 1;
            else
                return n * (n - 1) / 2;
            #endregion
        }
    }
}

Measured memory_bytes: 11220