(define nu (read-line))
(define ns (read-line))

(print ((lambda (b f) (if (>= b f)
                          (- b f)
                          -1))
       (* (list-ref (map string->number (string-split nu " ")) 0)
          (list-ref (map string->number (string-split nu " ")) 1))
       (fold + 0 (map string->number (string-split ns " ")))))