using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Linq;
using System.Numerics;

namespace AtCoderWorkspace
{
    public class Solver
    {
        public void Solve()
        {
            var cin = new Scanner();
            var a = cin.next().ToCharArray();
            var b = cin.next().ToCharArray();
            var c = cin.next().ToCharArray();

            var aq = new Queue<char>();
            var bq = new Queue<char>();
            var cq = new Queue<char>();
            for (int i = 0; i < a.Length; i++) aq.Enqueue(a[i]);
            for (int i = 0; i < b.Length; i++) bq.Enqueue(b[i]);
            for (int i = 0; i < c.Length; i++) cq.Enqueue(c[i]);

            var next = aq.Dequeue();
            var ans = ' ';
            while (true)
            {
                switch (next)
                {
                    case 'a':
                        if (aq.Count == 0)
                        {
                            ans = 'A';
                            goto A;
                        }
                        next = aq.Dequeue();
                        break;
                    case 'b':
                        if (bq.Count == 0)
                        {
                            ans = 'B';
                            goto A;
                        }
                        next = bq.Dequeue();
                        break;
                    case 'c':
                        if (cq.Count == 0)
                        {
                            ans = 'C';
                            goto A;
                        }
                        next = cq.Dequeue();
                        break;
                }
            }
            A:
            Console.WriteLine(""+ ans);
        }
    }

    class Scanner
    {
        string[] s;
        int i;

        char[] cs = new char[] { ' ' };

        public Scanner()
        {
            s = new string[0];
            i = 0;
        }

        public string next()
        {
            if (i < s.Length) return s[i++];
            string st = Console.ReadLine();
            while (st == "") st = Console.ReadLine();
            s = st.Split(cs, StringSplitOptions.RemoveEmptyEntries);
            if (s.Length == 0) return next();
            i = 0;
            return s[i++];
        }

        public int nextInt()
        {
            return int.Parse(next());
        }
        public int[] ArrayInt(int N, int add = 0)
        {
            int[] Array = new int[N];
            for (int i = 0; i < N; i++)
            {
                Array[i] = nextInt() + add;
            }
            return Array;
        }

        public int[] DPArrayInt(int N, int ex)
        {
            int[] Array = new int[N + ex];
            for (int i = 0; i < N; i++)
            {
                Array[i] = nextInt();
            }
            return Array;
        }

        public long nextLong()
        {
            return long.Parse(next());
        }

        public long[] ArrayLong(int N, long add = 0)
        {
            long[] Array = new long[N];
            for (int i = 0; i < N; i++)
            {
                Array[i] = nextLong() + add;
            }
            return Array;
        }

        public long[] DPArrayLong(int N, int ex)
        {
            long[] Array = new long[N + ex];
            for (int i = 0; i < N; i++)
            {
                Array[i] = nextLong();
            }
            return Array;
        }

        public double nextDouble()
        {
            return double.Parse(next());
        }


        public double[] ArrayDouble(int N, double add = 0)
        {
            double[] Array = new double[N];
            for (int i = 0; i < N; i++)
            {
                Array[i] = nextDouble() + add;
            }
            return Array;
        }
    }

    class Program
    {
        static void Main(string[] s) => new Solver().Solve();
    }
}


Measured memory_bytes: 13268