defmodule Main do
  def main do
    1..2
    |> Enum.map(fn _ ->
      IO.read(:line)
      |> String.split()
      |> Enum.map(&String.to_integer/1)
    end)
    |> solve
    |> IO.puts()
  end

  defp solve([[k, n, _], a]) do
    r = k * n - (a |> Enum.sum())

    cond do
      r < 0 -> -1
      true -> r
    end
  end
end