(define (extract-nth-digit a n current)
  (if (= current n)
      (modulo a 10)
      (extract-nth-digit (quotient a 10) n (+ 1 current))))


(define (length a)
  (if (and (< 0 a) (< a 10))
      1
      (+ 1 (length (/ a 10)))))

(define (middle-digit a)
  (if (even? (length a))
      -1
      (extract-nth-digit a (ceiling (/ (length a) 2)) 1)))