(defun main (&rest argv)
  (declare (ignorable argv))
  (let* ((query (read))
         (res 0d0))
    (dotimes (_ query)
      (let* ((x (read))
             (a (float (read) 0d0)))
        (declare (ignore x))
        (incf res a)))
    (princ (* res 3.1415926535897932384626433832795028841971693993751058209749445923078164062862089986280348253421170679d0))
    (terpri)))

(main)