System. Capabilites screenresolutionX

HI!

I’m all in perplexity,
I’m working within flash, and I’m just trying to get the size of my screen , apparently simply:

import flash.system.Capabilities;

trace(flash.system.capabilities.screenResolutionY);

>> flash doesn’t know “flash”, i’ve tried, not importing the class, uppercase and lowercase, .*, flash., fl., System.capabilities… what I am doing wrong?