private static void DisplayFirstNumber(string[] args)
{
   string arg = args.Length >= 1 ? args[0] : 
                              throw new ArgumentException("You must supply an argument");
   if (Int64.TryParse(arg, out var number))
      Console.WriteLine($"You entered {number:F0}");
   else
      Console.WriteLine($"{arg} is not a number.");                            
  
}