r,c = gets.split.map(&:to_i)
puts (r / 2) * (c / 2) * (r == c ? 1 : 2) + (r % 2) * (c / 2) + (r == c ? 0 : 1) * (r / 2) * (c % 2) + (r % 2) * ( c % 2) - 1