(defun main (&rest argv)
  (declare (ignorable argv))
  (let* ((n (read))
         (s (make-array n :element-type 'integer))
         (m -9876543210)
         (res 0))
    (dotimes (i n)
      (let* ((a (read))
             (b (read))
             (c (+ a (* b 4))))
        (setf (aref s i) c
              m (max m c))))
    (dotimes (i n)
      (when (/= (mod m 2) (mod (aref s i) 2))
            (format t "-1~%")
            (return-from main))
      (incf res (- m (aref s i))))
    (format t "~d~%" (floor res 2))))

(main)