; brush strokes

(define (f x)
  (do ((x x (cdr x)) (p 0 (car x))
       (s 0 (+ s (max (- (car x) p) 0))))
      ((null? x) s)))

(display (f '(1 4 3 2 3 1))) (newline)

(display (f '(4 1 2 1 2 2))) (newline)