problem passing argument to global-set-key

[Sarir Khamsi <sarir.khamsi@raytheon.com>]

I am trying to write a function that will grow or shrink a frame
using:

(defun sk-grow-frame (&optional n)
  "Grow or shrink the current frame."
  (interactive "p")
  (let ((my-arg n))
    (message (format "%d" my-arg)) ; some debug
    (set-frame-height nil (+ (screen-height) my-arg))))

and assigning it to some keys with:

(global-set-key (quote [C-M-up]) (quote sk-grow-frame))
(global-set-key (quote [C-M-down]) (quote sk-grow-frame -1))

The first keyboard shortcut works, ie, growing the frame by one, but
the second does not...it still increases the frame by one (instead of
decreasing it, which is what I want). What am I missing? I'm sure it's
something simple. Thanks.

Sarir