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

    if r != c
      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
    else
      if c.even?
        n = c/2
        puts (n/2.0 * (1.0 + (2 * n - 1))).to_i - 1
      else
        if c == 1
          puts 0
        else
          n = c/2
          puts (n/2.0 * (2.0 + 2.0 * n)).to_i
        end
      end
    end
  end
end

A.new