def tps(n,inCnt,out)
	s = []
	ret = []
	n.times do |i|
		s << i if inCnt[i] == 0
	end
	while v = s.shift
		ret << v
		#tの出力辺とその先のノードmを見て、辺を削除
		#mが入力辺を持っていない=mの最後の入力がtならm追加
		out[v].each do |m|
			inCnt[m] -= 1
			s << m if inCnt[m] == 0
		end
	end
	return ret
end

N,M = gets.split.map(&:to_i)
dp = [0]*(N+1)
cnt= [0]*(N+1)
cnt[0] = -1

vin = (N+1).times.map{[]}
vout = (N+1).times.map{[]}

M.times do
	x,y = gets.split.map(&:to_i)
	cnt[y] += 1
	vin[y] << x
	vout[x] << y
end

list = tps(N+1,cnt,vout)

list.each do |t|
	max = -1
	vin[t].each do |j|
		max = dp[j] if max < dp[j]
	end
	dp[t] = max+1
	#p [t,vin[t],dp[t]]
end
p dp.max