defmodule Math do
  def mul(_, 0) do
    0
  end
  def mul(0, _) do
    0
  end

  def mul(a, b) when b < 0 do
    sum = a + mul(a, b+1)
    sum*(-1)
  end

  def mul(a, b) do
    sum = a + mul(a, b-1)
    sum
  end
end