Santa's Little Helper

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

internal class Program
{
    static void Main()
    {
        int key = int.Parse(Console.ReadLine());
        var goodChildren = new List<string>();
        string input;
        while ((input = Console.ReadLine()) != "end")
        {
            string message = GetDecodedInput(input, key);
            string regex = @"[@](?<name>[A-Za-z]+)[^@!:>-]+[!](?<behaviour>[G|N])[!]";
            MatchCollection matchInfo = Regex.Matches(message, regex);

            if (matchInfo.Count == 0)
                continue;

            string childName = matchInfo[0].Groups["name"].Value;
            string childBehaviour = matchInfo[0].Groups["behaviour"].Value;

            if (childBehaviour == "G")
                goodChildren.Add(childName);
        }
        Console.WriteLine(String.Join("\n", goodChildren));
    }

    static string GetDecodedInput(string input, int key)
    {
        char[] decodedChars = input
            .ToCharArray()
            .Select(x => (char)(x - key))
            .ToArray();
        return String.Join("", decodedChars);
    }
}