Number 0...100 to Text

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

namespace Zero_to_100
{
    class Program
    {
        static void Main(string[] args)
        {
            var num = int.Parse(Console.ReadLine());
            var firstnum = num / 10;  // взема целочислената част - примерно 25/10 = 2
            var secnum = num % 10; // остатъка при деление на 10 - примерно 25%10= 5
            var bothnum = num; // Задаваме числото иначе няма да работи

            string secword = "";
            string firstword = "";
            string bothword = "";

            //Console.WriteLine(secnum);

            switch (secnum)
            {
                case 0: secword = "zero"; break;
                case 1: secword = "one"; break;
                case 2: secword = "two"; break;
                case 3: secword = "three"; break;
                case 4: secword = "four"; break;
                case 5: secword = "five"; break;
                case 6: secword = "six"; break;
                case 7: secword = "seven"; break;
                case 8: secword = "eight"; break;
                case 9: secword = "nine"; break;
                default: break;
            }
            switch (firstnum)
            {
                case 2: firstword = "twenty"; break;
                case 3: firstword = "thirty"; break;
                case 4: firstword = "fourty"; break;
                case 5: firstword = "fifty"; break;
                case 6: firstword = "sixty"; break;
                case 7: firstword = "seventy"; break;
                case 8: firstword = "eighty"; break;
                case 9: firstword = "ninety"; break;
                default: break;
            }
            switch (bothnum)
            {
                case 10: bothword = "ten"; break;
                case 11: bothword = "eleven"; break;
                case 12: bothword = "twelve"; break;
                case 13: bothword = "thirteen"; break;
                case 14: bothword = "fourteen"; break;
                case 15: bothword = "fifteen"; break;
                case 16: bothword = "sixteen"; break;
                case 17: bothword = "seventeen"; break;
                case 18: bothword = "eighteen"; break;
                case 19: bothword = "nineteen"; break;
                default: break;
            }

            if (num >= 0 && num <= 9)
            { Console.WriteLine(secword); }
            else if (num >= 10 && num <= 19)
            { Console.WriteLine(bothword); }
            else if (num >= 20 && num <= 100)
            {
                if (num % 10 == 0 && num != 10 && num != 100) // този израз описва числата 20 30 40 50 60 70 80 90 (без 10 и 100)
                { Console.WriteLine(firstword); }
                else if (num == 100)
                { Console.WriteLine("one hundred"); }
                else
                { Console.WriteLine(firstword + " " + secword); }

            }
            else
            { Console.WriteLine("invalid number"); }
        }
    }
}