import scala.io.StdIn.readLine

object Task3 extends App {
  val n = readLine().split(' ').map(_.toLong).head

  def eating(n: Long, k: Long) = {
    var total = n
    var Vasya = 0L
    var Petya = 0L

    while (total > 0L) {
      val eatVasya = if (total > k) k else total
      total = total - eatVasya

      val eatPetya = math.floor(total * 0.1).toInt
      total = total - eatPetya
      Vasya = Vasya + eatVasya
      Petya = Petya + eatPetya
    }

   Vasya > Petya
  }

  def sol(n: Long) = {
    var left = 1L
    var right = n
    while ( left != right) {
      val now = (left+right)/2
      if (!eating(n, now)) left = now + 1 else right = now
    }

    left
  }

  println(sol(n))
}