output_description: Print the length of the longest directed path in G.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N M
      x_1 y_1
      x_2 y_2
      :
      x_M y_M
attempt: N, M = map(int, input().split())
  XY = [list(map(int, input().split())) for i in range(M)]
  
  
  def tsort(N, E):
      c = [[] for i in range(N)]
      t = [0] * N
      for i, j in E:
          c[i].append(j)
          t[j] += 1
      A, B = zip(*E)
      s = set(range(N)) - set(B)
      l = []
      while s:
          n = s.pop()
          l.append(n)
          for i in c[n]:
              t[i] -= 1
              if t[i] == 0:
                  s.add(i)
      return c, l
  
  
  c, l = tsort(N, [(x - 1, y - 1) for x, y in XY])
  d = [0] * N
  for i in l:
      for n in c[i]:
          d[n] = max(d[n], d[i] + 1)
  print(max(d))
  
problem_description: 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.
samples: [{"input": "4 5\n    1 2\n    1 3\n    3 2\n    2 4\n    3 4", "output": "3\n    \n\nThe red directed path in the following figure is the longest:\n\n![](https://img.atcoder.jp/dp/longest_0_muffet.png)\n\n* * *"}, {"input": "6 3\n    2 3\n    4 5\n    5 6", "output": "2\n    \n\nThe red directed path in the following figure is the longest:\n\n![](https://img.atcoder.jp/dp/longest_1_muffet.png)\n\n* * *"}, {"input": "5 8\n    5 3\n    2 3\n    2 4\n    5 2\n    5 1\n    1 4\n    4 3\n    1 3", "output": "3\n    \n\nThe red directed path in the following figure is one of the longest:\n\n![](https://img.atcoder.jp/dp/longest_2_muffet.png)"}]