Adding only records that meet a criteria

Hello:

I have a spreadsheet with several columns in it.  The values that I want to add are in the columns.  I only want to add the columns where the heading for that column is "add".

Of course I can just choose those cells, but I am looking for a formula that will only add the values when the heading(or cell above them say "add")

I am attaching a small example.

Thank you! exampleofadd.xlsx