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.

N,M = map(int, input().split())

move = []
for i in range(M):
    move.append(list(map(int, input().split())))

dp = [0] * (N+1)
done_n = -1

def findpath(now_x):
    if now_x <= done_n:
        return dp[now_x]
    for x,y in move:
        if now_x == x:
            dp[now_x] = max(dp[now_x],findpath(y) + 1)
    return dp[now_x]

maxpath = 0
for do_n in range(1,N+1):
    done_n = do_n - 1
    maxpath = max(maxpath,findpath(do_n))
    print(dp)

print(maxpath)

import sys
sys.setrecursionlimit(10**5)


N,M = map(int, input().split())

move = {}
for i in range(M):
    x,y = map(int, input().split())
    move.setdefault(x,[]).append(y)

dp = [-1] * (N+10)

def findpath(now_x):
    if dp[now_x] > -1:
        return dp[now_x]
    maxpath = 0
    if now_x in move.keys():
        for y in move[now_x]:
            maxpath = max(maxpath,findpath(y) + 1)
    dp[now_x] = maxpath
    return maxpath

Ans = 0
for do_x in range(1,N+1):
    Ans = max(Ans,findpath(do_x))

print(Ans)
