Rotating camera with RMB drag

Hey guys, 4th post. I'm picking up more and more of Unity scripting as I'm going, and now I come asking not for advice on how to begin coding something, but rather advice making a minor correction to something I've created already.

The game mechanic in question is camera rotation - more specifically, orbiting the camera around the (perceived) center of the screen. It's to be a strategy-style camera. It already pans, and rotates correctly, although it previously rotated one direction by pressing and holding the RMB. The code that achieved this is as follows:

private var targetPosition : Vector3;

if (Input.GetMouseButtonDown(1))    {       targetPosition = transform.position;
targetPosition += transform.forward * 27; }   
}    if (Input.GetMouseButton(1))       transform.RotateAround(targetPosition,Vector3.up,50*Time.deltaTime);

Now while this worked wonderfully, I'd like the camera to rotateAround (only horizontally) by dragging the mouse with the RMB held down, left and right. The code below works, to a degree, but doesn't limit it to the horizontal axis, instead going (pardon the expression) 'mental' vertically and horizontally.

I just can't for the life of me find a way to restrict to the horizontal axis that doesn't result in a syntax error!

Here's the nearly-operable code:

var rmbDownPoint : Vector2;
var mouseRightDrag : boolean = false;
private var targetPosition : Vector3;

// Right Mouse Hold
if (Input.GetMouseButton(1)) {
    Screen.showCursor = false;
    } else {
    Screen.showCursor = true;
    //Move mouse left or right with RMB held
if (Input.GetMouseButtonDown(1))
    rmbDownPoint = Input.mousePosition;
    mouseRightDrag = true;

if (mouseRightDrag)
    var dragDifference : Vector2 = rmbDownPoint - Input.mousePosition;
    targetPosition += transform.forward * 27; 
    transform.RotateAround(targetPosition,Vector3.up+dragDifference ,50*Time.deltaTime);

    mouseRightDrag = false;

Ok, I've fixed it, incorporating 'Input.GetAxis' into the function.