Need help about random motion

i’m very2 new with flash.

so here’s the script from http://www.kirupa.com/developer/actionscript/random_motion.htm

i’ve changed it a little bit to fit my movie dimension

onClipEvent (load) {

//data you may want to change
width = 1050;
height = 300;
speed = Math.round(Math.random()*80)+1;
//initial positions
x = this._x=Math.random()*width;
y = this._y=Math.random()*height;
x_new = Math.random()*width;
y_new = Math.random()*height;

}
onClipEvent (enterFrame) {

//x movement
if (x_new>this._x) {

sign_x = 1;
} else {
sign_x = -1;

}
dx = Math.abs(x_new-this._x);
if ((dx>speed) || (dx<-speed)) {

this._x += sign_x*speed;
} else {
x_new = Math.random()*width;

}
//y movement
if (y_new>this._y) {

sign_y = 1;
} else {
sign_y = -1;

}
dy = Math.abs(y_new-this._y);
if ((dy>speed) || (dy<-speed)) {

this._y += sign_y*speed;
} else {
y_new = Math.random()*height;

}

}

but the thing is. i want it to move only around the region
width = from 850px tilll 1050px
and
height = from 100px to 300px

where do i change?

thanks in advance.