When putting the stuff on a web-page, I realized that the converter was correct only up to the number 4999. From 5000 one reuses the letters, but a line must be put on top to tell they are different from the small ones.
Another challenge was the convert-button. The button worked fine, but when I tried to hit enter, the page collapsed. After some research i found that I didn’t know much about the problems regarding forms/submit/enter. By now I do know a little more 😉
This is my Converter: