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: Accepted)

from collections import defaultdict

n,m=map(int,input().split())
adj=defaultdict(list)
deg=[0]*(n+1)
for i in range(m):
    x,y=map(int,input().split())
    adj[x].append(y)
    deg[y]+=1

dist=[0]*(n+1)

que=[]
for i in range(1,n+1):
    if deg[i]==0:
        que.append(i)

while que:
    v=que.pop()
    for i in adj[v]:
        deg[i]-=1
        if deg[i]==0:
            que.append(i)
        dist[i]=max(dist[i],dist[v]+1)

print(max(dist))