CONO-SUR 1989 Problem 3

A number $p$ is $perfect$ if the sum of its divisors, except $p$ is $p$ . Let $f$ be a function such that:

$f(n)=0$ , if n is perfect $f(n)=0$ , if the last digit of n is 4 $f(a.b)=f(a)+f(b)$

Find $f(1998)$