How the answer is 10? Please explained

Question

https://code.sololearn.com/cuCeGT1f8qvd/?ref=app

Answer 1

When precompiler process prod it will expand the expression in:
3+2*4-1
than result in 3+8-1=10 because multiplication has precedence respect addition/subtraction... For solve, define prod like

#define prod(i,j) (i)*(j)

So the expression will be expanded to:
(3+2)*(4-1)=5*3=15