Text rolling

Here is my problem. It should be quite easy but i don’t know much about action script.

I want to make a text rolling from 99 to 0 and then start again and again.
So I made a movie clip but i’m too lazy to write 99 , 98 , 97…

So I put a square for dynamic text (instance name ptchiffre) and an action layer.

That’s where i’m stuck…
I tried something like:

x = “100-y*5”;
ptchiffre = “x”

here y should be the frame number

logicaly the mc would have only 20 frame…

But if you know something easier to do it you are free to let me know