Sets & Dictionaries Advanced - 7. SoftUni Party

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

namespace Advanced
{
    class Program
    {
        static void Main(string[] args)
        {
            var VIP = new HashSet<string>();
            var regular = new HashSet<string>();

            var pattern = @"^[0-9][\w]+";

            var input = Console.ReadLine();


            while (input != "PARTY")
            {
                if (Regex.IsMatch(input, pattern))
                {
                    VIP.Add(input);
                }

                else
                {
                    regular.Add(input);
                }

                input = Console.ReadLine();
            }

            input = Console.ReadLine();

            while (input != "END")
            {
                if (VIP.Contains(input))
                {
                    VIP.Remove(input);
                }

                else
                {
                    regular.Remove(input);
                }
                input = Console.ReadLine();
            }

            Console.WriteLine(VIP.Count + regular.Count);

            foreach (var guest in VIP)
            {
                Console.WriteLine(guest);
            }

            foreach (var guest in regular)
            {
                Console.WriteLine(guest);
            }
        }
    }
}