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 PYTHON):

Solution:
import sys
sys.setrecursionlimit(10**6)

N,M=map(int, sys.stdin.readline().split())

nl=[ [] for i in range(N+1) ] 

for i in range(M):
	x,y=map(int, raw_input().split())
	nl[x].append(y)


memo={}

def depth(n):
	if n in memo: return memo[n]
	ret=0
	if len( nl[n] )==0:
		return 0
	else:
		for x in nl[n]:
			ret=max(ret, depth(x)+1)
	memo[n]=ret
	return ret


ans=0
for i in range(1,N+1):
	ans=max( ans , depth(i) )

print ans