I'm trying to see which columns are filled with data and then add up the
value above it.

For example:

A B C
1 500 400 300
2 1 2

---------------
In this case, I want to look at row 2 and see that column A and B are
filled, and thus sum up only column A and B of row 1. It seems like a simple
concept but I can't seem to figure out how to do this.

Thanks a lot in advance.