defmodule Main do 
  def main do
    IO.gets("") |> String.trim |> String.split |> Enum.map(&String.to_integer/1) |> pokky(0) |> IO.puts
  end
  
  def pokky([l, k], acc) when l-k*2 > 0 do
    pokky([l-k*2, k], acc+k)
  end
  
  def pokky([_, _], acc) do
    acc
  end
end