(define nu (read-line))
(define ns (read-line))

(print ((lambda (l n) (if (odd? n)
                          (list-ref l (div n 2))
                          (/ (+ (list-ref l (- (div n 2) 1)) (list-ref l (div n 2))) 2.0)))
        (sort (map string->number (string-split ns " "))) (string->number nu)))