using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {
        var input = Console.ReadLine().Split().Select(int.Parse).ToArray();
        var array = new List<int>[] { new List<int> { 0, 4, 8 }, new List<int> { 1, 5, 9 }, new List<int> { 2, 6, 10 }, new List<int> { 3, 6, 11 } };
        for (int i = 0; i < 4; i++)
        {
            var count = 0;
            foreach (var item in input)
            {
                if (array[i].Contains(item)) array[i].Remove(item);
            }

            if (array[i].Count == 1)
            {
                Console.WriteLine(array[i][0]);
                break;
            }
        }
    }
}