Using _x and _y

1

movement using _x and _y
hi,
following codes are for making a “circle” move along all four edgesof the screen…but some codes are missing…please add some more codes so that my circle(MC) finish its journey…1…go right…2 go down…3 go left and finally go up…to where it first started…
thanks

onClipEvent (load) {
this._x = 10;
this._y = 10;
}
onClipEvent (enterFrame)
{
if (this._x<400) {
this._x += 10;
} else {
this._y += 10;
}