using System;

public class Program

{

    public static void Main()

    {

        int? plant = null;

        Console.WriteLine((plant?.ToString().PadLeft(4, '0')));

        plant = 1;

        Console.WriteLine((plant?.ToString().PadLeft(4, '0')));

        plant = 10;

        Console.WriteLine((plant?.ToString().PadLeft(4, '0')));

        plant = 100;

        Console.WriteLine((plant?.ToString().PadLeft(4, '0')));

        plant = 1000;

        Console.WriteLine((plant?.ToString().PadLeft(4, '0')));

    }

}