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 (2.4.0) (judge verdict: Runtime Error)

from collections import defaultdict


def longest_path(n, edges):
    adj_list = defaultdict(list)
    for x, y in edges:  # x -> y
        adj_list[y].append(x)

    distances = [None] * n

    def dfs(x):
        if distances[x] is None:
            for y in adj_list[x]:
                dfs(y)
            distances[x] = max((distances[y] + 1 for y in adj_list[x]), default=0)

    for x in range(n):
        dfs(x)

    return max(distances)


def main():
    n, m = [int(x) for x in input().split()]
    edges = [None] * m
    for i in range(m):
        edges[i] = [int(x) - 1 for x in input().split()]

    return longest_path(n, edges)


print(main())