Powerpivot table add new measure

Hi all,
I am trying to add a simple calculated measure in power pivot table - not sure if this possible ...
but here is what i have ( table)
2 ----- na
10 ----- 4
20 ----- 10
16 ----- 4
want create a new measure from above table ( say column "c")

2 ---- na ---- 7
10 ----- 4 -----3
20 ----10 ---- 1

In normal excel formula here how you can get in column "C"..
( a2+b2)/a1 = c1 ,
(a3+b3/a2 = c2 and so an.


Please let me know the possibilities.

Thanks in advance

kevin