(define d (read))
(define (readn n)  (if (= n 0) '() (cons (let ( (a (read)) (b (read))) (- b a) ) (readn (- n 1)) ) ))
(let ( (l (readn d)))(if   (apply = l) (if (< (car l) 1) (print "-1") (print (car l))) (print "-1")))