r, c = gets.split.map(&:to_i)

m = case
when r.even? && c.even? then
  r == c ? r * c / 4 : r * c / 2
when r.odd? && c.odd? then
  r == c ? r * c / 4 + 1 : r * c / 2 + 1
else
  r * c / 2
end

puts m - 1