#| BEGIN (Write your solution here) |#
(define (fast-mul a b)
  (cond ((= b 0) 0)
        ((even? b) (double (fast-mul a (halve b))))
        (else (+ a (fast-mul a (- b 1))))))

(define (double n)
  (+ n n))

(define (halve n)
  (/ n 2))
#| END |#