(defparameter *N* (read))

(defun formula (a x y) (if (= a (- y x)) (- y x) -1))

(defun solve ()
    (let* ((x_1 (read)) (y_1 (read)) (a (if (> 0 (- y_1 x_1)) -1 (- y_1 x_1))))
    (dotimes (i (1- *N*)) (setq a (formula a (read) (read))))
    a))

(defun main () (format t "~A~%" (solve)))

(main)