import scala.collection.mutable

object Main extends App {

  import scala.io.StdIn._

  val Array(n, m) = readLine.trim.split(' ').map(_.toInt)
  val edges = Array.tabulate(n){_ ⇒ Nil: List[Int]}
  val inEdge = Array.tabulate(n){_ ⇒ 0}
  for (_ ← 0 until m){
    val Array(x, y) = readLine.trim.split(' ').map(_.toInt)
    edges(x - 1) ::= y - 1
    inEdge(y - 1) += 1
  }
  val longestPath = Array.tabulate(n){_ ⇒ n}
  val queue = mutable.PriorityQueue()(Ordering.by[(Int, Int), Int](_._2).reverse)
  inEdge.indices.filter(p ⇒ inEdge(p) == 0).foreach(p ⇒ queue.enqueue((p, n)))
  while (queue.nonEmpty){
    val (top, cost) = queue.dequeue()
    if (longestPath(top) == cost) {
      for (next ← edges(top)) {
        if (longestPath(next) > cost - 1) {
          longestPath(next) = cost - 1
          queue.enqueue((next, cost - 1))
        }
      }
    }
  }
  println(n - longestPath.min)
  implicit class Extend[T](value: T){
    def also(func: T ⇒ Unit): T = {func(value); value}
    def let[A](func: T ⇒ A): A = func(value)
  }
}