I am not sure I fully understand what you are subtracting. But I know
for sure how to get the expression R1+R2, then R3+R4 etc, thus you can
include the following expression in your formula

OFFSET('Sheet1'!$A$1,2*(ROW()-1),0)+OFFSET('Sheet1'!$A$1,2*(ROW()-1)+1,0)

and then subtract as necessary.

HTH
Kostis Vezerides