Problem: 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.

Examples

Input

4 5
1 2
1 3
3 2
2 4
3 4


Output

3


Input

6 3
2 3
4 5
5 6


Output

2


Input

5 8
5 3
2 3
2 4
5 2
5 1
1 4
4 3
1 3


Output

3

Solution (in PYTHON3):

Solution:
import sys

sys.setrecursionlimit(1000000)


def dfs(v):
    if cache[v] > -1:
        return cache[v]
    lv = links[v]
    cache[v] = ret = max(map(dfs, lv)) + 1 if lv else 0
    return ret


n, m = map(int, input().split())
sss = set(range(n))
links = [set() for _ in range(n)]
cache = [-1] * n
for line in sys.stdin.readlines():
    x, y = map(int, line.split())
    x -= 1
    y -= 1
    sss.discard(y)
    links[x].add(y)

print(max(map(dfs, sss)))