Problem with Reactive Extension Observers

Question

I was working on an app using Reactive Extensions and got into the following problem:

say i have two observers P and Q, i want to build a third observer R that if two values of P comes without a Q, R outputs 0. And if after a P comes a Q, R outputs the result of a method passing those values, something like:

P0    Q0    ->    R0 = f(P0,Q0)    
P1          ->    R1 = 0    
P2    Q1    ->    R2 = f(P2,Q1)    
P3          ->    R3 = 0    
P4          ->    R4 = 0    
P5    Q2    ->    R5 = f(P5,Q2)
(...)

and the values come into the obsevers in the following order:

P0 Q0 P1 P2 Q1 P3 P4 P5 Q2

thanks for your help.