using System;
namespace yukicoder
{
    class Program
    {

        static void Main(string[] args)
        {
            var B = Console.ReadLine();
            int sum = 0 , max = 0;
            int i,j;
            var sp = B.Split(" ");

            for(i = 0; i < sp.Length; i++)
            {
                sum += int.Parse(sp[i]);
            }   
            for(j = 1; j <= sp.Length + 1; j++)
            {
                max += j;
            }
            int ans = max - sum;
            Console.WriteLine(ans); 
        }
    }
}