(define (mame K N lis)
  (let* ((result (- (* K N) (fold + 0 lis))))
    (if (< result 0) -1
        result)))

(define (create-list n)
  (if (= n 0) '()
      (cons (read) (create-list (- n 1)))))

(define (main args)
  (let* ((K (read))
         (N (read))
         (F (read)))
    (print (mame K N (create-list F)))) 0)