How to get Absolute value at the parent node when 2 more lowest nodes have the -ve value assigned to

If the 2 out of 4 L0 nodes have -ve number say -10 then the parent node should give me abs value as 40 and not 20. Please suggest a way forward

Ideal scenario -

P -> M1 -> L01, L02,

       M2 -> L03, L04

L01 = -10

L02 = 10

L03 = 10

L04 = -10

P = 20

When we consider absolute value it should show me

P =40

How can apply absolute func at "P" and get 40 out of it without going to each lowest node to check for -ve numbers.