using System;

public class Program

{

    public static void Main()

    {

        int? val = null;

        Console.WriteLine($"1: {val}");

        val++;

        Console.WriteLine($"2: {val}");

        val = 3;

        Console.WriteLine($"3: {val}");

        val++;

        Console.WriteLine($"4: {val}");

    }

}