# frozen_string_literal: true

def solve
  if R == C
    if R.odd?
      (R / 2) * (R / 2 + 1)
    else
      R**2 / 4 - 1
    end
  else
    Rational(R * C, 2).ceil - 1
  end
end

R, C = gets.chomp.split.map(&:to_i)

puts solve