class A
  def initialize
    r, c = gets.chomp.split(' ').map(&:to_i)

    if c.even?
      puts (r * (c/2)) - 1
    else
      if r.even?
        puts (r * (c/2) + (r/2)) - 1
      else
        puts (r * (c/2) + (r/2)+1) - 1
      end
    end
  end
end

A.new