(defvar N)
(setq N (parse-integer (read-line)))
(defvar X)
(defvar Y)

(defvar temp)
(setq temp (read-line))
(setq temp (read-from-string (concatenate 'string "(" temp ")" )))
(setq X (nth 0 temp))
(setq Y (nth 1 temp))

;(print X)
;(print Y)
(defvar ans)
(setq ans (- Y X))
(if (< ans 0) (progn (format t "-1~%") (quit)))

(loop for i from 2 to N do
        (setq temp (read-line))
        (setq temp (read-from-string (concatenate 'string "(" temp ")" )))
        (setq X (nth 0 temp))
        (setq Y (nth 1 temp))
        (if (not (= (- Y X) ans))  (progn (format t "-1~%") (quit)))
)
(format t "~d~%" ans)