(defun split (x str)
  (let ((pos (search x str))
        (size (length x)))
    (if pos
      (cons (subseq str 0 pos)
            (split x (subseq str (+ pos size))))
      (list str))))

(setq *n* (read))
(setq *ai* (mapcar #'parse-integer (split " " (read-line))))

(print (float (/ 1 (apply #'+ (mapcar #'(lambda (a) (/ 1 a)) *ai*)))))