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())
  edge=[[] for _ in range(n)]
  deg=[0] * n
  for i in range(m):
      x, y =map(int, input().split())
      edge[x - 1 ].append(y - 1)
      deg[y - 1] +=1 
  
  from collections import deque
  
  que =deque()
  for i in range(n):
      if deg[i] == 0:
          que.append(i)
  dp = [0] *n
  
  while que:
      v = que.popleft()
      nv = edge[v]
      for nv in nv:
          deg[nv] -=1
          if deg[nv] ==0:
              que.append(nv)
              dp[nv] = max(dp[v] + 1 ,dp[nv])
  print(max(dp)
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)"}]