(defvar K)
(defvar N)
(defvar F)
(defvar A)
(defvar temp)
(setq temp (read-line))
(setq temp (concatenate 'string "(" temp ")" ))
(setq temp (read-from-string temp))
(setq K (nth 0 temp))
(setq N (nth 1 temp))
(setq F (nth 2 temp))

(setq temp (read-line))
(setq temp (concatenate 'string "(" temp ")" ))
(setq temp (read-from-string temp))
(setq A temp)

(defvar sum)
(setq sum 0)
(loop for x in A do (setq sum (+ sum x)))
;(format 't "<~d ~d ~d ~%>" K N F)
(if (=< sum (* K N)) (format 't "~d~%" (- (* K N) sum)) (format 't "-1~%"))