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
  for i in family_age_list:
    if i < all_bean_grains:
      all_bean_grains -= i
      eat_count += 1
    else:
      break
  if eat_count != len(family_age_list):
    print(-1)
  else:
    print(all_bean_grains)

if __name__ == '__main__':
  main()