1 to 100 in Words

using System;

namespace _100_in_Words
{
    class Program
    {
        static void Main(string[] args)
        {
            var intNumber = int.Parse(Console.ReadLine());
            string number = intNumber.ToString();
            string numberText = null;

            if (intNumber < 0 || intNumber > 100)
                Console.WriteLine("invalid number");
            else if (number == "100")
                Console.WriteLine("one hundred");
            else
            {
                int numberLength = number.Length;

                if (numberLength == 2)
                {
                    switch (number[0])
                    {
                        case '1':
                            if (number[1] == '0')
                            {
                                numberText = "ten";
                            }
                            else if (number[1] == '1')
                            {
                                numberText = "eleven";
                            }
                            else if (number[1] == '2')
                            {
                                numberText = "twelve";
                            }
                            else if (number[1] == '3')
                            {
                                numberText = "thirteen";
                            }
                            else if (number[1] == '4')
                            {
                                numberText = "fourteen";
                            }
                            else if (number[1] == '5')
                            {
                                numberText = "fifteen";
                            }
                            else if (number[1] == '6')
                            {
                                numberText = "sixteen";
                            }
                            else if (number[1] == '7')
                            {
                                numberText = "seventeen";
                            }
                            else if (number[1] == '8')
                            {
                                numberText = "eighteen";
                            }
                            else if (number[1] == '9')
                            {
                                numberText = "nineteen";
                            }
                            break;
                        case '2':
                            numberText = "twenty";
                            break;
                        case '3':
                            numberText = "thirty";
                            break;
                        case '4':
                            numberText = "forty";
                            break;
                        case '5':
                            numberText = "fifty";
                            break;
                        case '6':
                            numberText = "sixty";
                            break;
                        case '7':
                            numberText = "seventy";
                            break;
                        case '8':
                            numberText = "eighty";
                            break;
                        case '9':
                            numberText = "ninety";
                            break;
                    }
                    if (number[0] != '1')
                    {
                        switch (number[1])
                        {
                            case '1':
                                numberText += " one";
                                break;
                            case '2':
                                numberText += " two";
                                break;
                            case '3':
                                numberText += " three";
                                break;
                            case '4':
                                numberText += " four";
                                break;
                            case '5':
                                numberText += " five";
                                break;
                            case '6':
                                numberText += " six";
                                break;
                            case '7':
                                numberText += " seven";
                                break;
                            case '8':
                                numberText += " eight";
                                break;
                            case '9':
                                numberText += " nine";
                                break;
                        }
                    }
                }
                else
                {
                    switch (number)
                    {
                        case "0":
                            numberText = "zero";
                            break;
                        case "1":
                            numberText = "one";
                            break;
                        case "2":
                            numberText = "two";
                            break;
                        case "3":
                            numberText = "three";
                            break;
                        case "4":
                            numberText = "four";
                            break;
                        case "5":
                            numberText = "five";
                            break;
                        case "6":
                            numberText = "six";
                            break;
                        case "7":
                            numberText = "seven";
                            break;
                        case "8":
                            numberText = "eight";
                            break;
                        case "9":
                            numberText = "nine";
                            break;
                    }
                }

                Console.WriteLine(numberText);
            }
        }
    }
}