Incrementing values to a variable

i’m trying to add values to a variable…
i have this:

var myMC:MovieClip = new MovieClip();;, 0, 400, 300);;
myMC.x = myMC.y = 00;

myMC.addEventListener(MouseEvent.MOUSE_DOWN, startMotionHandler);

function startMotionHandler($evt:MouseEvent):void {

myMC.addEventListener(MouseEvent.MOUSE_DOWN, drag);
function drag(e:MouseEvent){

myMC.addEventListener(MouseEvent.MOUSE_UP, drop);
function drop(e:MouseEvent){
myMC.x = myMC.y = 00;


a simple drag and drop on a MC.
i would like to store the X displacement when i drag. let say i have 30 in a variable wich store x position of myMC, if i drag the MC 200 units the variable should read 230 and so on. i hope my explanation is clear.
any help?? don’t have a clue on this.

*it should be done at runtime.