How to add limiting "walls" to movement of MC controlled by buttons

[tiGRAN=-2001]

Hello wizards,

I am trying to make a simple scrolling thumbnail menu for some photos.

I used the following script (tutorial) to add the movement functions to the up and down buttons, which have instance names "moveUp" and '"moveDown".

The thumbnails are in a vertical looking MC named "rocket".

I have no problem scrolling up and down, but I want to limit the functionality of the buttons when the rocket is between coordinates Y=0 and Y=-1200

How can I add that to this script?

moveUp.onPress = function() {
rocket.onEnterFrame = function() {
this._y -= 18;
}
}
moveUp.onRelease = function() {
delete rocket.onEnterFrame;
}
//
moveDown.onPress = function() {
rocket.onEnterFrame = function() {
this._y -= -18;
}
moveDown.onRelease = function() {
delete rocket.onEnterFrame;
}
//
moveUp.onReleaseOutside = function() {
delete rocket.onEnterFrame;
}
moveDown.onReleaseOutside = function() {
delete rocket.onEnterFrame;
}

I actually tried to use the following code to stop the movement but it doesn't work because you can pass the limit assigned and it only gets back after another click.

//
if (rocket._y > 0) {
rocket._y = 0;
}
}
if (rocket._y < -1200) {
rocket._y = -1200;
}

Please help me find a solution,

Thanks!!
Tigran