Offsetting formulas for consecutive data sets
 

I need to create a formula that is easily "drag-downable" of the
following form - say I have a column of numbers and i want to sum each
2 like this :

1 6
5 15
3 21
12
15
6


So that the 6 is 1+5, the 7 is 3+12, 11 is 15+6. At the moment I have
this formula :

=SUM(OFFSET($A$1,ROW(1:1)*2-2,0,2,1))

which works but was wondering if there was a more easy / obvious way to
do this ... ?

Many thanks,
Francis.