using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace template
{
    class Program
    {
        static void Main(string[] args)
        {
            int[] x = Console.ReadLine().Split().Select(int.Parse).ToArray();
            for (int i = 0; i < 9; i++)
            {
                if (x[i] != i + 1)
                {
                    Console.WriteLine(i + 1);
                    break;
                }
            }
        }
    }
}