NumAsWordsHelp

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

namespace NumAsWordsHelp
{
    class Program
    {
        static void Main(string[] args)
        {
            var number = int.Parse(Console.ReadLine());
            string posDec = "";
            string posOne = "";
            string posTeen = "";
            string numWord = "";
            if(number >= 0 && number < 100)
            {
                // Number BETWEEN 0 and 99 [0..99]
                var digitDEC = number / 10;
                var digitONE = number % 10;

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

                if(number == 0)
                {
                    numWord = posOne;
                }
                else
                {
                    if(digitDEC == 1)
                    {
                        numWord = posTeen;
                    }
                    else
                    {
                        if(number > 19)
                        {
                            switch(digitONE)
                            {
                                case 0:
                                    numWord = posDec;
                                    break;
                                default:
                                    numWord = posDec + " " + posOne;
                                    break;
                            }
                        }
                        else
                        {
                            if(digitDEC == 0)
                            {
                                numWord = posOne;
                            }
                        }

                    }
                }
                Console.WriteLine(numWord);
            } // If [0..99]
            else
            {
                if(number == 100)
                {
                    Console.WriteLine("one hundred");
                }
                else
                {
                    Console.WriteLine("invalid number");
                }
            }

        }
    }
}