flash mx 2004-need global variables to get value only when application starts

[hike101]

hi.
i have 4 layers.
the first and main layer is called "map".
i gave declared a few global vars and everytime i return to frame1 in "map" the vars get reassigned the startup values, for example:
_global Myvar=1;
function abc{
_global.Myvar=5;
}

when the frame goes back to 1 the Myvar gets the interger 1 instead remaining 5. (assuming i called function abc on a nother frame).
so my question is how do i get global vars to get initialized only when the app loads for the first time?

thanks