ThumbPosition Slider value reverse

i need use slider in reversed position 
when is max left it's max value(255) and when it go to max right it's min value (0)

can some one help me with this ??



Perhaps .....

Have you read the AI2 free online eBook   http://appinventor.mit.edu/explore/ai2/tutorials.html   ... the links are at the bottom of the Web page?

It has a chapter on math operations and another on logic.

thx