How to stop formulae changing when inserting a new column

Hi, the data below is a simplified version of the sheet I am working with.

--------------------wk29----wk28
----A--------B--------C--------D
--=B1-C1----10-------3--------2
--=B2-C2----11-------4--------1

I insert a new column inbetween B and C every week but then the formula in col A changes to B1-D1 etc. I want it to show the column A value minus the current week data i.e column C.

How can I make the formula so that column A always show B1-C1 even if I insert another column?

Thank you.

Replace formula in A1 with

=B1-INDIRECT("C"&ROW(A1))

or

=B1-INDEX(1:1,,3)


the latter being non-volatile while the former is volatile

Thank you, that worked just great!!! once I'd figured out how it works.