HELP with extending functionality of this prog.

[wavo7]

Hey all, I could really use your help. I'm super stuck. I'm writing a program that will except a number (positive or negative) from the user, and the display the number in written form to them. EX. 124 the computer will display one hundred twenty four. I need to account for negatives too. The problem is I have it working fine from 0 to 99, but I'm having a brain freeze on extending it to one billion. I need to account for numbers that high, it really shouldn't be too hard, but I'm having issues. Any help would be GREATLY appreciated, I need to finish this SOON. Thanks so much!

Here's the code I have so far:
This code should run on your machine if you have Java installed, sorry about the wrapping of the text, I hope you can make sense of it all.

import java.io.*;
import java.util.*;
import java.lang.*;


public class WordNumbers
{
public static void main(String[] args) throws IOException
{
final String quitString = "done";
String inputString;
int firstNumber, counter;

BufferedReader input =
new BufferedReader(new InputStreamReader(System.in));

System.out.println("Enter "+quitString+" when finished"

String numberWords[] = {"one","two","three","four","five","six","seven",
"eight","nine","ten","eleven","twelve","thirteen",
"fourteen","fifteen","sixteen","seventeen","eighteen",
"nineteen"};
String tenWords[] = {"twenty","thirty","fourty","fifty","sixty","seventy",
"eighty","ninety"};
String bigWords[] = {"hundred","thousand","million","billion","trillion"};

inputString = input.readLine();

while (!inputString.equals(quitString))
{
firstNumber = Integer.parseInt(inputString);

if(firstNumber < 0)
{
System.out.println(&quot;negative &quot; + numberWords[firstNumber - 1]);
}

while (firstNumber > 0)
{
if(firstNumber >=1000000 && (firstNumber <=9999999))
{
System.out.println(bigWords[2]);
}
else
if(firstNumber >= 1000 && (firstNumber <= 9999))
{
System.out.println(bigWords[1]);
}
else
if(firstNumber > 99 && (firstNumber <= 999))
{
System.out.println(bigWords[0]);
}
else
if(firstNumber < 20)
{
System.out.println(&quot;You entered &quot; + numberWords[firstNumber - 1]);
}
else
{
int unitsNo = firstNumber % 10 ;
int decNo = firstNumber/10;
int unit2 = firstNumber % 100;
int dec2 = firstNumber/100;

if( unitsNo == 0 )
{
System.out.println(&quot;You entered &quot; + tenWords[ decNo - 2 ]);
}

if( unitsNo > 0)
{
System.out.println(&quot;You entered &quot; + tenWords[ decNo - 2 ] + &quot; &quot; + numberWords[ unitsNo - 1]);
}
else
if( unitsNo > 9)
{
System.out.println(&quot;hahah&quot;
}
}
firstNumber = -1;
}

if(firstNumber == 0)
{
System.out.println(&quot;You entered zero&quot;
}
inputString = input.readLine();
}
}
}