(define (make-point x y) (cons x y))

(define (x-point p) (car p))
(define (y-point p) (cdr p))

(define (make-segment start end) (cons start end))
(define (start-segment s) (car s))
(define (end-segment s) (cdr s))

(define (mid-point s)
  (let ((start-x (x-point (start-segment s)))
        (start-y (y-point (start-segment s)))
        (end-x   (x-point (end-segment s)))
        (end-y   (y-point (end-segment s))))
    (make-point
      (average start-x end-x)
      (average start-y end-y))))

(define (print-point p)
  (display "(")
  (display (x-point p))
  (display ",")
  (display (y-point p))
  (display ")")
  (newline))

(define (average x y)
  (/ (+ x y) 2.0))