Problem:
Score : 100 points

Problem Statement

There is a directed graph G with N vertices and M edges.
The vertices are numbered 1, 2, \ldots, N, and for each i (1 \leq i \leq M), the i-th directed edge goes from Vertex x_i to y_i.
G does not contain directed cycles.

Find the length of the longest directed path in G.
Here, the length of a directed path is the number of edges in it.

Constraints

All values in input are integers.

2 \leq N \leq 10^5

1 \leq M \leq 10^5

1 \leq x_i, y_i \leq N

All pairs (x_i, y_i) are distinct.

G does not contain directed cycles.

Input

Input is given from Standard Input in the following format:

N M
x_1 y_1
x_2 y_2
:
x_M y_M

Output

Print the length of the longest directed path in G.

Sample Input 1

4 5
1 2
1 3
3 2
2 4
3 4

Sample Output 1

3

The red directed path in the following figure is the longest:

Sample Input 2

6 3
2 3
4 5
5 6

Sample Output 2

2

The red directed path in the following figure is the longest:

Sample Input 3

5 8
5 3
2 3
2 4
5 2
5 1
1 4
4 3
1 3

Sample Output 3

3

The red directed path in the following figure is one of the longest:

Solution in Scala (2.11.7) (judge verdict: Wrong Answer)

import java.io._
import java.util.StringTokenizer
import scala.util.control.Breaks.{breakable,break}

object Main extends App {
  val in = new InputReader(System.in)
  val pw = new PrintWriter(System.out)

  val N, M = in.next().toInt
  val graph = Array.fill[List[Int]](N+1)(Nil)
  for(_ <- 1 to M) {
    val a, b = in.next().toInt
    graph(a) = b :: graph(a)
  }


  //
  val dp = Array.fill[Int](N+1)(-1)
  val stack = new scala.collection.mutable.Stack[Int]

  def calc(): Unit = {
    while(!stack.isEmpty) {
      val now = stack.pop()
      if(now < 0) {
        if(dp(-now) != -1){
          var res = 0
          for(e <- graph(-now)){
            res = Math.max(res, dp(e)+1)
          }
          dp(-now) = res
        }
      }
      else {
        stack.push(-now)
        for(e <- graph(now)){
          if(dp(e) == -1) {
            stack.push(e)
          }
        }
      }
    }
  }

  
  for (i <- 1 to N) {
    if(dp(i) == -1){
      stack.push(i)
      calc()
    }
  }


  println(dp.max)
}


class InputReader(stream: InputStream) {
  private val reader = new BufferedReader(new InputStreamReader(stream))
  private var tokenizer: StringTokenizer = new StringTokenizer(reader.readLine())
  def next(): String = {
    while (!tokenizer.hasMoreTokens()) {
      tokenizer = new StringTokenizer(reader.readLine())
    }
    tokenizer.nextToken()
  }
}