(define (iota n . args)
  (let ((start (if (pair? args) (car args) 0))
        (step  (if (and (pair? args) (pair? (cdr args))) (cadr args) 1)))
    (let loop ((m n) (last (+ start (* step (- n 1)))) (a '()))
      (if (zero? m)
          a
	  (loop (- m 1) (- last step) (cons last a))))))

(define (sum a)
  (if (null? a) 0 (+ (car a) (sum (cdr a)))))

(define (solve K N F A)
  (max -1 (- (* K N) (sum A))))

(display (let ((K (read)) (N (read)) (F (read)))
	   (let ((A (map (lambda (x) (read)) (iota F))))
	     (solve K N F A))))
(newline)