using System;

public class Program

{

    public static void Main()

    {

        int[] input = [3,0,1];

        float sumofNaturalNumber = input.Length * (input.Length +1) /2 ;

        int sumofInput = 0;

        foreach(int i in input)

        {

            sumofInput += i;

        }

        Console.WriteLine(sumofNaturalNumber - sumofInput);

    }

}