Exam\Song Encryption

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


namespace Orders
{
    class Program
    {
        static void Main(string[] args)
        {
string input = string.Empty;

while((input = Console.ReadLine()) != "end")
{
    string artist = input.Substring(0, input.IndexOf(":"));
    string song = input.Substring(input.IndexOf(":") + 1);

    if (ValidArtist(artist) && ValidSong(song))
    {

        int length = artist.Length;

        StringBuilder sb = new StringBuilder();

        for (int i = 0; i < input.Length; i++)
        {
            if (input[i] == ':')
            {
                sb.Append('@');
            }
            else if (input[i] == ' ' || input[i] == '\'')
            {
                sb.Append(input[i]);
            }
            else
            {
            char symbol = (char)(input[i] + length);

            if (char.IsUpper(input[i]) && symbol > 'Z')
            {
                symbol = (char)(symbol - 26);
            }
            else if (char.IsLower(input[i]) && symbol > 'z')
            {
                symbol = (char)(symbol - 26);
            }
            sb.Append(symbol);
            }


        }

        Console.WriteLine($"Successful encryption: {sb}");
    }
    else
    {
        Console.WriteLine("Invalid input!");
    }
}
        }

        public static bool ValidArtist(string artist)
        {
            bool isValid = true;

            if(!char.IsUpper(artist[0]))
            {
                isValid = false;
            }

            for (int i = 1; i < artist.Length; i++)
            {
                if (!char.IsLower(artist[i]) && artist[i] != '\'' && artist[i] != ' ')
                {
                    isValid = false;
                    break;
                }
            }


            return isValid;
        }

        public static bool ValidSong(string song)
        {
            bool isValid = true;

            for (int i = 0; i < song.Length; i++)
            {
                if(!char.IsUpper(song[i]) && song[i] != ' ')
                {
                    isValid = false;
                    break;
                }
            }


            return isValid;
        }
    }
}