Calculating Alternating Values

I have a bunch of start End times in columns calculating total time :-

=SUMPRODUCT((ISEVEN(COLUMN(C3:AD3))-ISODD(COLUMN(C3:AD3)))*C3:AD3)

Which works great except is there an easy way to adapt this incase someone inserts a column and thevalues start in an even row?

Or is there a better way?