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

namespace yukicoder
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var inputs = Console.ReadLine().Split().Select(int.Parse).ToArray();

            for (int i = 0; i < inputs.Length; i++)
            {
                if (inputs[i] != i + 1)
                {
                    Console.WriteLine(i+1);
                    return;
                }
            }
            Console.WriteLine(10);
        }
    }
}