Consider a Boolean function \( f(w,x,y,z) \) such that $f(w,0,0,z) = 1 $$f(1,x,1,z) = x + z $$f(w,1,y,z) = wz + y $
The number of literals in the minimal sum-of-products expression of \( f \) is \(\underline{\hspace{2cm}}\).