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

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

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