p03166 Educational DP Contest - Longest Path
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.

Examples

Input

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


Output

3


Input

6 3
2 3
4 5
5 6


Output

2


Input

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


Output

3
```python
import sys
sys.setrecursionlimit(1000000)
 
n, m = map(int, input().split())
g = [[] for _  in range(n)]
for _ in range(m):
  x, y = map(int, input().split())
  x, y = x -1, y-1
  g[x].append(y)  
 
# dp[v]: ノードvを起点にしたときの、Gの有効パスの長さの最大値
dp = [-1]*n
def rec(v):
  if dp[v] != -1:
    return dp[v]
  else:
    res = 0
    for u in g[v]:
      res = max(res, rec(u)+1)
    dp[v] = res
    return dp[v]
  
for i in range(n):
  rec(i)
print(max(dp))
```