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:
from sys import setrecursionlimit
setrecursionlimit(1<<30)
def f(x):
  if z[x]==-1:
  	z[x]=max([f(i) for i in v[x]]+[-1])+1
  return z[x]
n,m=map(int,raw_input().split())
v=[[] for _ in xrange(n)]
for x,y in [map(int,raw_input().split()) for _ in xrange(m)]:
  v[x-1].append(y-1)
z=[-1]*n
for i in xrange(n):
  if z[i]==-1:
    f(i)
print max(z)