(defun main (&rest argv)
  (declare (ignorable argv))
  (let* ((c (read))
         (a (read))
         (b (read))
         (b+a (+ b a))
         (b-a (- b a))
         (v (* 0.25d0 pi pi b+a b-a b-a)))
    (format t "~f~%" (* v c))))

(main)