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
sys.setrecursionlimit(10**6)
def f(x):
	global hashi
	if(x in hashi):
		return hashi[x]
	if(adj[x]==[]):
		return 0
	mino=0
	for i in adj[x]:
		mino=max(mino,f(i))
	hashi[x]=mino+1
	return mino+1
l=input().split()
n=int(l[0])
m=int(l[1])
adj=[[] for i in range(n+1)]
for i in range(m):
	l=input().split()
	u=int(l[0])
	v=int(l[1])
	adj[u].append(v)
maxa=0
hashi=dict()
for i in range(1,n+1):
	maxa=max(maxa,f(i))
print(maxa)