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 Ruby (2.3.3) (judge verdict: Runtime Error)

require 'tsort'

class Hash
  include TSort
  alias tsort_each_node each_key
  def tsort_each_child(node, &block)
    fetch(node).each(&block)
  end
end

N,M = gets.split.map(&:to_i)
dp = [0]*(N+1)
vOut = Hash[N.times.map{|i| [i+1,[]]}]
vIn = Hash[N.times.map{|i| [i+1,[]]}]

M.times do
	x,y = gets.split.map(&:to_i)
	vOut[x] << y
	vIn[y] << x
end
tps = vOut.tsort.reverse
N.times do |i|
	t = tps[i]
	max = -1
	vIn[t].each do |j|
		max = dp[j] if max < dp[j]
	end
	dp[t] = max+1
	#p [t,vIn[t],dp[t]]
end
p dp.max