a, b = gets.split.map(&:to_i)
if a == b
    if a % 2 == 0
        puts a * a / 4 - 1 
    else
        puts (a - 1) ** 2 / 4 + (b + 1) / 2 - 1 
    end
elsif a % 2 == 0
    puts a * b / 2 - 1
else
    puts (a - 1) * b / 2 + (b + 1) / 2 - 1
end