(define nu (read-line))
(define ns (read-line))
(print (let ((b (* (list-ref (map string->number (string-split nu " ")) 0)
                   (list-ref (map string->number (string-split nu " ")) 1)))
             (f (fold + 0 (map string->number (string-split ns " ")))))
  (if (> b f)
      (- b f)
      -1)))