class Yukicoder
  def initialize
    @r, @c = gets.chomp.split.map(&:to_i)
  end

  def run
    k = (@r >> 1) * (@c >> 1)

    k *= 2 if @r != @c
    k += @c >> 1 if @r.odd?
    k += @r >> 1 if @c.odd? && @r != @c
    k += 1 if @r.odd? && @c.odd?

    return k - 1
  end
end

puts Yukicoder.new.run