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)


n,m=map(int,input().split())

p=[[] for _ in range(n)]

for _ in range(m):
    x,y=map(int,input().split())
    p[x-1].append(y-1)

f=[-1 for _ in range(n)]

def calc(x):
    if f[x]!=-1:
        return f[x]
    if len(p[x])==0:
        f[x]=0
        return 0
    f[x]=max([calc(i) for i in p[x]])+1
    return f[x]

m=0
for i in range(n):
    if f[i]!=-1:
        continue
    m=max(m,calc(i))
print(m)