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)

from collections import defaultdict
import sys
sys.setrecursionlimit(10**7)
v,e=map(int,input().split())
m=defaultdict(list)

for i in range(e):
    x,y=map(int,input().split())
    m[x].append(y)
dp=defaultdict(lambda:-1)
def longestPath():
    
    def findLongestPath(vtx):
        if(dp[vtx]!=-1):
            return dp[vtx]
        res=0
        for i in m[vtx]:
            res=max(res,1+findLongestPath(i))
        dp[vtx]=res
        return res
        
        
    
    ans=0
    for i in range(1,v+1):
        ans=max(ans,findLongestPath(i))
    
    return ans;

print(longestPath())