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(10**6)
def dfs(v):
    seen[v]=1
    for nv in G[v]:
        if seen[nv]:
            continue
        dfs(nv)
    topo.append(v)

N,M=map(int,input().split())
G=[[] for i in range(N)]
for i in range(M):
    x,y=map(lambda x:int(x)-1,input().split())
    G[x].append(y)
seen=[0]*N
topo=[]
for i in range(N):
    if not seen[i]:
        dfs(i)
dp=[0]*N
for v in reversed(topo):
    for nv in G[v]:
        dp[nv]=max(dp[nv],dp[v]+1)
print(max(dp))