## problem with formula

Hi
I just made some formula for some parameters
i drawed a circle and i used constraint to set it's length. i right clicked on it and selected "edit formula"
i selected my needed parameter and clicked ok. After that , i saw know change !! the the length of circle doesn't change after using parameter with formula ...
Do you know what's the problem ?!
to see what's happening exactly , i recorded my problem , please watch this :