05. Parking Validation

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;

namespace ConsoleApp8
{
    class Program
    {
        static void Main(string[] args)
        {
            int numberOfCommands = int.Parse(Console.ReadLine());
            Dictionary<string, string> usernamePlate = new Dictionary<string, string>();
            for (int i = 0; i < numberOfCommands; i++)
            {
                string inputLine = Console.ReadLine();
                var elements = inputLine.Split(' ').ToArray();
                string command = elements[0];
                string username = elements[1];
                if (command == "register")
                {
                    string licensePlate = elements[2];
                    //АА1234BB is Valid Plate
                    bool isValidPlate = licensePlate.Length == 8
                        && char.IsUpper(licensePlate[0])
                        && char.IsUpper(licensePlate[1])
                        && char.IsUpper(licensePlate[7])
                        && char.IsUpper(licensePlate[6])
                        && char.IsDigit(licensePlate[2])
                        && char.IsDigit(licensePlate[3])
                        && char.IsDigit(licensePlate[4])
                        && char.IsDigit(licensePlate[5]);
                    if (isValidPlate)
                    {
                        if (!usernamePlate.ContainsKey(username))
                        {
                            if (!usernamePlate.ContainsValue(licensePlate))
                            {
                                usernamePlate.Add(username, licensePlate);
                                Console.WriteLine($"{username} registered {licensePlate} successfully");
                            }
                            else
                            {
                                Console.WriteLine($"ERROR: license plate {licensePlate} is busy");
                            }
                        }
                        else
                        {
                            Console.WriteLine($"ERROR: already registered with plate number {usernamePlate[username]}");
                        }
                    }
                    else
                    {
                        Console.WriteLine($"ERROR: invalid license plate {licensePlate}");
                    }
                }
                else
                {
                    if (!usernamePlate.ContainsKey(username))
                    {
                        Console.WriteLine($"ERROR: user {username} not found");
                    }
                    else
                    {
                        Console.WriteLine($"user {username} unregistered successfully");
                        usernamePlate.Remove(username);
                    }
                }
            }
            foreach (var item in usernamePlate)
            {
                Console.WriteLine($"{item.Key} => {item.Value}");
            }
        }
    }
}