Number 0...100 to Text

using System;



class Program
{
    static void Main()
    {

        int n = int.Parse(Console.ReadLine());
        int firstNumber = n / 10;
        int secondNumber = 0;
        int bothNumbers = n;
        if (n >= 0 && n <= 10)
        {
            secondNumber = n;
        }
        else if (n >= 20)
        {
            secondNumber = n % 10;
        }
        string firstWord = "";
        string secondWord = "";
        string bothWords = "";

        switch (firstNumber)
        {
            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;
            case 10:
                firstWord = "one hundred";
                break;
            default:
                break;
        }

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

        switch (bothNumbers)
        {
            case 10:
                bothWords = "ten";
                break;
            case 11:
                bothWords = "eleven";
                break;
            case 12:
                bothWords = "twelve";
                break;
            case 13:
                bothWords = "thirteen";
                break;
            case 14:
                bothWords = "fourteen";
                break;
            case 15:
                bothWords = "fifteen";
                break;
            case 16:
                bothWords = "sixteen";
                break;
            case 17:
                bothWords = "seventeen";
                break;
            case 18:
                bothWords = "eighteen";
                break;
            case 19:
                bothWords = "nineteen";
                break;
            default:
                break;
        }

        if (n % 10 == 0 && n != 0)
        {
            secondWord = "";
        }

        if (n >= 0 && n < 10)
        {
            Console.WriteLine(secondWord);
        }

        else if (n >= 10 && n <= 19)
        {
            Console.WriteLine(bothWords);
        }
        else if (n >= 20 && n <= 100)
        {
            if (n % 10 == 0)
            {
                Console.WriteLine(firstWord);
            }
            else
            {
                Console.WriteLine(firstWord + " " + secondWord);
            }

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

    }
}