C# book - 16. Number 0...100 to Text

using System;
namespace _16._Number_0_100_to_Text
{
    class Program
    {
        static void Main(string[] args)
        {
            int num = int.Parse(Console.ReadLine());

            if (num > 100 || num < 0)
            {
                Console.WriteLine("invalid number");
                return;
            }

            int n1 = num % 10;
            int n10 = ((num - n1) / 10) % 10;

            string text1 = ""; // за единици
            string text10 = ""; // за десетици
            string text11 = ""; // за десетици като eleven, twelve...(няма единици)
            string text100 = ""; // за сто

            switch (n1)
            {
                case 0: text1 = "zero"; break;
                case 1: text1 = "one"; break;
                case 2: text1 = "two"; break;
                case 3: text1 = "three"; break;
                case 4: text1 = "four"; break;
                case 5: text1 = "five"; break;
                case 6: text1 = "six"; break;
                case 7: text1 = "seven"; break;
                case 8: text1 = "eight"; break;
                case 9: text1 = "nine"; break;
                default:
                    break;
            }

            if (num / 10 < 10)
            {
                if (n10 == 1)
                {
                    if (n1 == 0) text11 = "ten";
                    else if (n1 == 1) text11 = "eleven";
                    else if (n1 == 2) text11 = "twelve";
                    else if (n1 == 3) text11 = "thirteen";
                    else if (n1 == 4) text11 = "fourteen";
                    else if (n1 == 5) text11 = "fifteen";
                    else if (n1 == 6) text11 = "sixteen";
                    else if (n1 == 7) text11 = "seventeen";
                    else if (n1 == 8) text11 = "eightteen";
                    else if (n1 == 9) text11 = "nineteen";
                }
                else if (n10 == 2)
                {
                    if (n1 == 0) text11 = "twenty";
                    else
                    {
                        text10 = "twenty";
                    }
                }
                else if (n10 == 3)
                {
                    if (n1 == 0) text11 = "thirty";
                    else
                    {
                        text10 = "thirty";
                    }
                }
                else if (n10 == 4)
                {
                    if (n1 == 0) text11 = "forty";
                    else
                    {
                        text10 = "forty";
                    }
                }
                else if (n10 == 5)
                {
                    if (n1 == 0) text11 = "fifty";
                    else
                    {
                        text10 = "fifty";
                    }
                }
                else if (n10 == 6)
                {
                    if (n1 == 0) text11 = "sixty";
                    else
                    {
                        text10 = "sixty";
                    }
                }
                else if (n10 == 7)
                {
                    if (n1 == 0) text11 = "seventy";
                    else
                    {
                        text10 = "seventy";
                    }
                }
                else if (n10 == 8)
                {
                    if (n1 == 0) text11 = "eighty";
                    else
                    {
                        text10 = "eighty";
                    }
                }
                else if (n10 == 9)
                {
                    if (n1 == 0) text11 = "ninety";
                    else
                    {
                        text10 = "ninety";
                    }
                }
            }
            else text100 = "one hundred";


            if (text100 == "one hundred")
            {
                Console.WriteLine(text100);
            }
            else if (text11 == "" && text10 == "")
            {
                Console.WriteLine(text1);
            }
            else if (text11 == "")
            {
                Console.WriteLine($"{text10} {text1}");
            }
            else
            {
                Console.WriteLine(text11);
            }
        }
    }
}