(defmacro calc (l)
  `(float (/ (+ (car ,l) (cadr ,l)) 2)))

(defun alchemy (l)
  (if (= (length l) 2)
      (calc l)
      (let ((result (calc l))
            (tmp l))
        (pop tmp)
        (pop tmp)
        (push result tmp)
        (alchemy tmp))))

(defun main ()
  (let* ((N (read))
         (v-list (loop for i from 1 to N
                       collect (read))))
    (sort v-list #'<)
    (print (alchemy v-list))))

(main)