(define (iota n . args)
  (let ((start (if (pair? args) (car args) 0))
        (step  (if (and (pair? args) (pair? (cdr args))) (cadr args) 1)))
    (let loop ((m n) (last (+ start (* step (- n 1)))) (a '()))
      (if (zero? m)
          a
	  (loop (- m 1) (- last step) (cons last a))))))

(define (min-element a)
  (if (null? a)
      1001001001
      (min (car a) (min-element (cdr a)))))

(define (max-element a)
  (if (null? a)
      -1001001001
      (max (car a) (max-element (cdr a)))))

(define (unique? a)
  (= (min-element a) (max-element a)))
    

(define (solve a)
  (let ((b (map (lambda (x) (- (cadr x) (car x))) a)))
    (if (unique? b)
	(if (positive? (car b))
	    (car b)
	    -1)
	-1)))
  
(display (solve (let ((N (read))) (map (lambda (x) (list (read) (read))) (iota N)))))
(newline)