using System;
class Program
    {
        static void Main(string[] args)
        {
            string s = Console.ReadLine();
            string[] t = s.Split(' ');
            for (int i = 0; i < 9; ++i)
            {
                if ( int.Parse(t[i]) != (i+1)) {
                    Console.WriteLine("{0}", i+1);
                    break;
                }
            }
        }
    }