(let* ((l1 (string-split (read-line) " "))
       (l2 (string-split (read-line) " "))
       (k (string->number (car l1)))
       (n (string->number (cadr l1)))
       (f (string->number (caddr l1)))
       (a (map string->number l2))
       (s (fold (lambda (x y) (+ x y)) 0 a))
       (r (- (* k n) s)))
 (print (if (negative? r) -1 r)))