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 Python (3.4.3) (judge verdict: Accepted)

# python template for atcoder1
import sys
sys.setrecursionlimit(10**9)
input = sys.stdin.readline

N, M = map(int, input().split())
edges = [set() for _ in range(N)]
ins = [0]*N
for _ in range(M):
    a, b = map(lambda x: int(x)-1, input().split())
    ins[b] += 1
    edges[a].add(b)

dp = [-1]*N


def dfs(s):
    if dp[s] != -1:
        return dp[s]
    mx = 0
    if edges[s]:
        mx = max([dfs(dest)+1 for dest in edges[s]])
    dp[s] = mx
    return mx


ans = -1
for i in range(N):
    ans = max(ans, dfs(i))

print(ans)