defmodule Main do
  def main do
    beans_sum = IO.gets("") |> String.trim |> String.split |> Enum.take(2) |> Enum.map(&String.to_integer/1) |> List.foldl(1, fn x, acc -> x * acc end)
    age_sum   = IO.gets("") |> String.trim |> String.split |> Enum.map(&String.to_integer/1) |> Enum.sum
    out(beans_sum, age_sum)
  end
  
  def out(beans_sum, age_sum) when beans_sum >= age_sum do
    IO.puts beans_sum-age_sum
  end
  
  def out(_, _) do
    IO.puts -1
  end
end