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

class Program
{
    static void Main (string[] args)
    {
        var input = Console.ReadLine().Split().Select(int.Parse);
        var oneToTen = Enumerable.Range(1, 10);

        var diff = input.Except(oneToTen).ToArray();
        Console.WriteLine(diff[0]);
    }
}