inputs = STDIN.readlines.map(&:chomp)
l, k = inputs[0].split(/\s+/).map(&:to_i)

def yuu_length(l, k)
  if l % (2 * k) == 0
    (l / (2 * k) - 1) * k
  else
    (l / (2 * k)) * k
  end
end

puts yuu_length(l, k)