(use scheme.base)
(use scheme.read)
(use scheme.write)
(use gauche.base)
(use srfi.152)


(define yuki9013
  (let*
    (
      (ubh (map string->number (string-split (read-line) ",")))
      (u (car ubh))
      (b (car (cdr ubh)))
      (h (car (cdr (cdr ubh))))
    )
    (format #t "~,2f~%" (/ (* (+ u b) h) 2))
  )
)