air application position on desktop

hi there,
i have an application(air) i want to load in the center of my desktop.
the application has a fixed size.
If i start my application it is not loaded in the center of my screen, the bottom of my application runs out of the screen.

so my question; How to calculate the desktop sizes and set the application on a specific point?
for example top right or bottom left.

Greetz, jacob