#!/usr/bin/env gosh

(define (str->lst str)
   (map (^x (string->number x))
	(string-split str " "))
  )

(define (main _)
  (let* ([bns (apply * (take (str->lst (read-line)) 2))]
	 [num (apply + (str->lst (read-line)))]
	 [rmng (- bns num)])
    (display (if (negative? rmng) -1 rmng))
    )
  (newline)
  0)