def main():

  eat_count = 0
  bean_grains, pick_packs, family_menbers = map(int, input().split())
  family_age_list = list(map(int, input().split()))
  all_bean_grains = bean_grains * pick_packs
  total_family_age = sum(family_age_list)
  leftover_beans = all_bean_grains - total_family_age
  if leftover_beans >= 0:
    print(leftover_beans)
  else:
    print(-1)

if __name__ == '__main__':
  main()