Exception forumla

Hi there,

I was wondering if anyone could help me with a forumla I need. I know it should be possible to do, but as an Excel novice I'm not too sure how to go about it.

I would like to add the total of an entire column. However, if a value appears in another column, I would like that value to be added instead. So, for instance, I would like to add everything up in Column A. However, if there is a value in Column B, I would like the forumla to ignore the value in A and add the value in Column B instead. How can I go about doing this?

Any help is much appreciated!