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)

import sys
input = sys.stdin.readline
sys.setrecursionlimit(10**7)
n, m = map(int, input().split())
edges = [[] for _ in range(n)]
for _ in range(m):
    x, y = list(map(lambda x: int(x)-1, input().split()))
    edges[x].append(y)


def memoize(f):
    memo = dict()

    def memoized(*args):
        if args in memo.keys():
            return memo[args]
        else:
            memo[args] = f(*args)
            return memo[args]
    return memoized

@memoize
def longest_path(v):
    retval = 0
    for v2 in edges[v]:
        retval = max(retval, longest_path(v2)+1)
    return retval


ans = 0
for i in range(n):
    ans = max(ans, longest_path(i))
print(ans)