$0 = 0
$1 = N / K
$1 = $1 + 100  # 二分探索上限
$2 = $1 - 1
while $0 < $2
  $3 = $0 + $1
  $3 = $3 / 2  # x
  $4 = N * K
  $4 = $4 + $3  # x + NK
  $4 = $4 * $3  # x(x+NK)
  $5 = N * N
  if $4 <= $5
    $0 = $3
  end
  if $4 > $5
    $1 = $3
    $2 = $1 - 1
  end
end
$0 = N + $0
return $0