Performing an action on a selection (ActiveCell?)

Hello everyone,

I'm a bit of a beginner in excel macros and I have a small problem. I would like to perform an action (multiply by (-1) to get opposite numbers) on an "active selection".

Its easy to make a macro to do this on a single active cell:
ActiveCell.Value = ActiveCell.Value * (-1)

....But what I want to do is perform this same action on all the cells I have selected. Is there some function to do this? I have tried searching around the board and on the internet but havent found anything to help me out with this specific thing.

I have attached a picture to show what im trying to do. The macro I have written only does the action on the 1st cell instead of all the selected ones.

What I have atm is:

Sub Opposite()

ActiveCell.Value = ActiveCell.Value * (-1)

End Sub

Thanks in advance,
Glooms