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:
#!/usr/bin/env python2
"""
This file is part of https://github.com/cheran-senthil/PyRival
Copyright 2019 Cheran Senthilkumar <hello@cheran.io>

"""
from __future__ import division, print_function

import itertools
import os
import sys
from atexit import register
from io import BytesIO


class dict(dict):
    """dict() -> new empty dictionary"""

    def items(self):
        """D.items() -> a set-like object providing a view on D's items"""
        return dict.iteritems(self)

    def keys(self):
        """D.keys() -> a set-like object providing a view on D's keys"""
        return dict.iterkeys(self)

    def values(self):
        """D.values() -> an object providing a view on D's values"""
        return dict.itervalues(self)


def gcd(x, y):
    """greatest common divisor of x and y"""
    while y:
        x, y = y, x % y
    return x


range = xrange

filter = itertools.ifilter
map = itertools.imap
zip = itertools.izip

sys.stdin = BytesIO(os.read(0, os.fstat(0).st_size))
sys.stdout = BytesIO()
register(lambda: os.write(1, sys.stdout.getvalue()))

input = lambda: sys.stdin.readline().rstrip('\r\n')

sys.setrecursionlimit(10**6)


def main():
    n, m = map(int, input().split())

    graph = [[] for _ in range(n)]
    for i in range(m):
        x, y = map(int, input().split())
        graph[x - 1].append(y - 1)

    dp = [-1] * n
    for node, children in enumerate(graph):
        if children == []:
            dp[node] = 0

    def solve(x):
        for child in graph[x]:
            if dp[child] == -1:
                solve(child)

        dp[x] = 1 + max(dp[child] for child in graph[x])

    for i in range(n):
        if dp[i] == -1:
            solve(i)

    print(max(dp))


if __name__ == '__main__':
    main()