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Is it possible to reduce canvas size by having a set width or height removed from the center of the image and having the remaining sides collapse in together?

For example, say I have a 800x600 image, and I want to remove a 800x100 horizontal slice from the middle, thus reducing the image dimensions to 800x500.

Is there a (reasonably short) way of doing something like this in Photoshop (CS4)?

Thanks