using System;

public class Program

{

    public static void Main()

    {

        int number;

            string word = "";

            string convertNumber;

​

            Console.WriteLine("Number to words");

            Console.WriteLine("Input a number to convert: ");

            convertNumber = Console.ReadLine();

​

            if (Int32.TryParse(convertNumber, out number) == false)

            {

                Console.WriteLine("Invalid number. Please input a positive integer.");

            }

            else

            {

                if (number == 1000)

                {

                    Console.WriteLine("One thousand");

                }

                else if (number == 0)

                {

                    Console.WriteLine("Zero");

                }

                else

                {

                    if (number >= 100)

                    {

                        switch (number / 100)

                        {

                            case 1:

                                word = "One hundred";

                                break;

                            case 2:

                                word = "Two hundred";

                                break;

                            case 3:

                                word = "Three hundred";

                                break;

                            case 4:

                                word = "Four hundred";

                                break;

                            case 5:

                                word = "Five hundred";

                                break;

                            case 6:

                                word = "Six hundred";

                                break;

                            case 7:

                                word = "Seven hundred";

                                break;

                            case 8:

                                word = "Eight hundred";

                                break;

                            case 9:

                                word = "Nine hundred";

                                break;

                        }

                        number = number % 100;

​

                    if (number >= 21)

                        switch (number / 10)

                        {

                            case 2:

                                word = word + " twenty";

                                break;

                            case 3:

                                word = word + " thirty";

                                break;

                            case 4:

                                word = word + " fourty";

                                break;

                            case 5:

                                word = word + " fifty";

                                break;

                            case 6:

                                word = word + " sixty";

                                break;

                            case 7:

                                word = word + " seventy";

                                break;

                            case 8:

                                word = word + " eighty";

                                break;

                            case 9:

                                word = word + " ninety";

                                break;

                        }

                    number = number % 10;

​

                    if (number >= 10)

                        switch (number - 10)

                        {

                            case 1:

                                word = word + " eleven";

                                break;

​

                            case 2:

                                word = word + " twelve";

                                break;

                            case 3:

                                word = word + " thirteen";

                                break;

                            case 4:

                                word = word + " fourteen";

                                break;

                            case 5:

                                word = word + " fifteen";

                                break;

                            case 6:

                                word = word + " sixteen";

                                break;

                            case 7:

                                word = word + " seventeen";

                                break;

                            case 8:

                                word = word + " eighteen";

                                break;

                            case 9:

                                word = word + " nineteen";

                                break;

​

                        }

                    number = number - 10;

                    }

​

                    switch (number)

                    {

                        case 1:

                            word = "one";

                            break;

                        case 2:

                            word = "two";

                            break;

                        case 3:

                            word = "three";

                            break;

                        case 4:

                            word = "four";

                            break;

                        case 5:

                            word = "five";

                            break;

                        case 6:

                            word = "six";

                            break;

                        case 7:

                            word = "seven";

                            break;

                        case 8:

                            word = "eight";

                            break;

                        case 9:

                            word = "nine";

                            break;

                    }

                    Console.WriteLine(word);

                }

            }

    }

}