(define (System.out.println x)
  (begin
    (display x)
    (newline)
  )
)

(let (
      (foo (read))
     )
  (let loop((X (read)) (Y (read)) (Ans -1))
    (if (eof-object? X)
        (System.out.println Ans)
        (if (> (- Y X) 0) 
           (if (and (>= Ans 0) (not (= Ans (- Y X))))
               (System.out.println -1)
               (loop (read) (read) (- Y X))
           )
       (System.out.println -1)
      )
    )
  )
)