Problem: 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

Solution (in PYTHON3):

Solution:
import sys
from heapq import *
sys.setrecursionlimit(200000)

def rec(i):
  if dp[i] != 0:
    return dp[i]
  res = 0
  for j in E[i]:
    res = max(rec(j) + 1, res)
  dp[i] = res
  return dp[i]


#入力
N,M = (int(x) for x in input().split())
#エッジ
E = [[] for _ in [0]*N]
dp = [0]*N

for i in range(M):
  a,b = (int(x) for x in input().split())
  E[a-1].append(b-1)

ans = 0
for i in range(N):
  ans = max(ans, rec(i))

#出力
print(ans)