Excel: sum odd/even rows only in R1C1 -

How can I combine cells using lines 1/1 or even lines?

I tried to follow the example. Suppose you have a table like this:

  | 1 | 6 | | 2 | | 3 | Then a 
  1]: r [2] c [-1]), 2) = 1, rc [-1]: r [2] c [ -1], 0))   
 Even though it should give  4    
 
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 I -  shift  - enter  < / Kbd>, this will give the desired results.