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:
from collections import deque

n, m = [int(i) for i in input().split()]
X = [[int(i) for i in input().split()] for j in range(m)]

E = [[] for i in range(n+1)]
IN = [0] * (n+1)

for a, b in X:
    E[a].append(b)
    IN[b] += 1

q = deque((i, 0) for i in range(1, n+1) if IN[i] == 0)
D = [0] * (n+1)

while q:
    i, d = q.popleft()
    IN[i] -= 1
    if IN[i] > 0:
        continue
    D[i] = d
    for e in E[i]:
        q.append((e, d+1))

print(max(D))