#lang sicp

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

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

(define (average a b)
  (/ (+ a b) 2.0))

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