using System;

namespace ConsoleApp5
{
    class Program
    {
        static void Main()
        {
            string[] input =Console.ReadLine().Split(' ');
            int cnt = 1;
            foreach(string aa in input) {
                if (cnt != Convert.ToInt32(aa))
                {
                    Console.Write(cnt);
                    return;
                }
                else { cnt++; }
            }
        }
    }
}