using System;
using System.Linq;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            var line1 = Console.ReadLine().Split(' ').Select(c => int.Parse(c)).ToArray();

            var b = line1;
            int result = 0;

            for (int i = 0;i < b.Length ; i++)
            {
                if (b[i] != i + 1)
                {
                    result = i + 1;
                    break;
                }
            }

            //結果を表示
            Console.WriteLine(result);
        }
    }
}