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

public class Yuki693
{
	public static void Main()
	{
		var n = int.Parse(Console.ReadLine());
		var a = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToList();
		
		Console.WriteLine(Math.Abs((n + 1) * n /2) - a.Sum());
	}
}