OCR for a very simple CAPTCHA

I would like a C# class I can implement in my own project to recognize the 4 numbers in a captcha I will supply. Suggested interface: public string ReadCaptcha(Bitmap captchaImage); The CAPTCHA always consists of exactly 4 numbers between 0 and 9, the background is always white, and numbers do not overlap. It is a relatively simply CAPTCHA. Your code may not use any 3d party dll's. Your code must be able to read the captcha with at least 90% accuracy and in less than 1-2 seconds (in an average spec system) Please see the attached sample CAPTCHAS before bidding.