l, k = lk.split(' ').map(&:to_i)
if k == 1
  if l / 2 < 1
    puts 0
  else
    puts (l / 2) - 1
  end
else
  k2 = k * 2
  times = l / k2
  if times == 1
    times = 0 if l % k2 == 0
  end
  puts times * k
end