Problem:
Score : 100 points

Problem Statement

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.

Sample Input 1

4 5
1 2
1 3
3 2
2 4
3 4

Sample Output 1

3

The red directed path in the following figure is the longest:

Sample Input 2

6 3
2 3
4 5
5 6

Sample Output 2

2

The red directed path in the following figure is the longest:

Sample Input 3

5 8
5 3
2 3
2 4
5 2
5 1
1 4
4 3
1 3

Sample Output 3

3

The red directed path in the following figure is one of the longest:

Solution in PyPy3 (7.3.0) (judge verdict: Accepted)

import sys
# import re
import math
import collections
# import decimal
import bisect
import itertools
import fractions
# import functools
import copy
# import heapq
import decimal
# import statistics
import queue

# import numpy as np

sys.setrecursionlimit(10 ** 9)
INF = 10 ** 16
MOD = 10 ** 9 + 7
# MOD = 998244353

ni = lambda: int(sys.stdin.readline())
ns = lambda: map(int, sys.stdin.readline().split())
na = lambda: list(map(int, sys.stdin.readline().split()))


# ===CODE===

def main():
    n, m = ns()
    e = [[] for _ in range(n)]
    for _ in range(m):
        a, b = ns()
        e[a - 1].append(b - 1)

    res = [-1 for _ in range(n)]

    def dfs(ti):
        tmp = 0
        for vi in e[ti]:
            if res[vi] != -1:
                tmp = max(tmp, res[vi] + 1)
            else:
                tmp = max(tmp, dfs(vi) + 1)
        res[ti] = tmp
        return tmp

    ans = 0
    for i in range(n):
        if res[i] == -1:
            ans = max(ans, dfs(i))

    print(ans)


if __name__ == '__main__':
    main()