#| BEGIN (Write your solution here) |#
(define (f n)
  (if (< n 3)
      n
      (+ (f (- n 1)) 
         (* 2 (f (- n 2))) 
         (* 3 (f (- n 3))))))
(define (f-iter n)
  (if (< n 3)
      n
      (helper 2 1 0 n)))

(define (helper a b c count)
  (if   (< count 3)
        a
        (helper (+ a (* 2 b) (* 3 c)) a b (- count 1))))
#| END |#