5. Parking Validation

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

class Program
{
    static void Main()
    {
        int linesCount = int.Parse(Console.ReadLine());
        Dictionary<string,string> registeredUsers = new Dictionary<string, string>();

        for (int i = 0; i < linesCount; i++)
        {
            string input = Console.ReadLine();
            string action = input.Split()[0];

            if (action == "register")
            {
                RegisterUser(input, registeredUsers);
            }
            else if (action == "unregister")
            {
                UnRegisterUser(input, registeredUsers);
            }
        }

        foreach (var registeredUser in registeredUsers)
        {
            Console.WriteLine($"{registeredUser.Key} => {registeredUser.Value}");
        }
    }

    private static bool LicensePlateIsValid(string licensePlate)
    {
        if (licensePlate.Length != 8)
        {
            return false;
        }

        string middle = licensePlate.Substring(2, 4);
        string ends = licensePlate.Substring(0, 2) + licensePlate.Substring(licensePlate.Length - 2, 2);

        int value = 0;
        bool endsAreLetters = ends.All(l => l >= 65 && l <= 90);
        bool middleIsNumbers = int.TryParse(middle, out value);

        if (!(endsAreLetters && middleIsNumbers))
        {
            return false;
        }

        return true;
    }

    private static void RegisterUser(string input, Dictionary<string, string> registeredUsers)
    {
        string username = input.Split()[1];
        string licensePlate = input.Split()[2];

        if (LicensePlateIsValid(licensePlate))
        {
            if (registeredUsers.ContainsKey(username))
            {
                Console.WriteLine($"ERROR: already registered with plate number {registeredUsers[username]}");
            }
            else if (registeredUsers.ContainsValue(licensePlate))
            {
                Console.WriteLine($"ERROR: license plate {licensePlate} is busy");
            }
            else
            {
                Console.WriteLine($"{username} registered {licensePlate} successfully");
                registeredUsers[username] = licensePlate;
            }
        }
        else
        {
            Console.WriteLine($"ERROR: invalid license plate {licensePlate}");
        }
    }

    private static void UnRegisterUser(string input, Dictionary<string, string> registeredUsers)
    {
        string username = input.Split()[1];
        if (!registeredUsers.ContainsKey(username))
        {
            Console.WriteLine($"ERROR: user {username} not found");
        }
        else
        {
            Console.WriteLine($"user {username} unregistered successfully");
            registeredUsers.Remove(username);
        }
    }
}