import strutils,sequtils,math

let knf: seq[int] = stdin.readline.split.map(parseint)
let mame_per_bag: int = knf[0]
let bag: int = knf[1]
let num_member: int = knf[2]

let aaa: seq[int] = stdin.readline.split.map(parseint)

if mame_per_bag * bag - aaa.sum >= 0:
  echo mame_per_bag * bag - aaa.sum
else:
  echo -1