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: Wrong Answer)

N, M = gets.split.map(&:to_i)

adjacency_matrix = Array.new(N) { Array.new(N, false) }
dp = Array.new(N) { Array.new(N, nil) }

M.times do
  x, y = gets.split.map(&:to_i)
  dp[x - 1][y - 1] = 1
  adjacency_matrix[x - 1][y - 1] = true
end

max = 0

N.times do |k|
  dp_k = dp[k]

  N.times do |x|
    dp_x = dp[x]
    next if dp_x[k].nil?

    N.times do |y|
      next if dp_k[y].nil?
      tmp = dp_x[k] + dp_k[y]

      if dp_x[y].nil? || dp_x[y] < tmp
        dp_x[y] = tmp
        max = tmp
      end
    end
  end
end

puts max