Adding values in an array between 2 specified matches

Hi 

I am working on a cost forecasting spreadsheet. I want to sum the costs over a specified period from a start week to an end week. The data is in the forma of a table with week numbers on the top and costs in rows for different departments.

I can get the index for the specific columns that match the week numbers I want, but how do i use that or something else to only sum the costs over the period specified by the 2 week numbers

Many Thanks

Francis