C#: Convert number to words.

I’m not a quite programmer in C# but thanks to someone from Google and stackoverflow forums and he gave me some clues about our activity.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace WindowsFormsApplication2
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        public static string NumberToWords(int number)
        {
            string words = "";

            if ((number / 1000) > 0)
            {
                words += NumberToWords(number / 1000) + " thousand ";
                number %= 1000;
            }

            if ((number / 100) > 0)
            {
                words += NumberToWords(number / 100) + " hundred ";
                number %= 100;
            }

            if (number > 0)
            {
                if (words != "")
                    words += "and ";

                var unitsMap = new[] { "zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen" };
                var tensMap = new[] { "zero", "ten", "twenty", "thirty", "forty", "fifty", "sixty", "seventy", "eighty", "ninety" };

                if (number < 20)
                    words += unitsMap[number];
                else
                {
                    words += tensMap[number / 10];
                    if ((number % 10) > 0)
                        words += "-" + unitsMap[number % 10];
                }
            }

            return words;
        }
        
        private void button1_Click(object sender, EventArgs e)
        {
            int number = Int32.Parse(richTextBox1.Text);
            string word = NumberToWords(number);

            richTextBox2.Text = word;
        }
    }
}