#| BEGIN (Write your solution here) |#
(define (f n)
  (cond ((< n 3) n)
        (else (+ (f (- n 1))
               (* 2 (f (- n 2)))
               (* 3 (f (- n 3)))))))

(define (f-iter n)
  (f-iterr 2 1 0 n))

(define (f-iterr a b c count)
  (if (= count 0)
      c
      (f-iterr (+ a (* 2 b) (* 3 c)) 
               a b (- count 1))))
#| END |#