import itertools as iter
import collections as coll
import heapq as hq
import bisect as bis
from decimal import Decimal as dec
from copy import deepcopy as dcopy
import math
import sys
sys.setrecursionlimit(10**6)
def input():
    return sys.stdin.readline().rstrip()
def getN():
	return int(sys.stdin.readline())
def getNs():
	return map(int,sys.stdin.readline().split())
def getList():
	return list(map(int,sys.stdin.readline().split()))
def strinps(n):
	return [sys.stdin.readline().rstrip() for _ in range(n)]
pi = 3.141592653589793
mod = 10**9+7
MOD = 998244353
INF = math.inf
dx = [1,0,-1,0];	dy = [0,1,0,-1]

"""
強連結成分分解（SCC）
from : https://github.com/ryusuke920/kyopro_educational_90_python/blob/main/solve_python/021.py
ref : https://manabitimes.jp/math/1250
"""

class SCC:
	def __init__(self, n):
		self.n = n
		self.graph = [[] for _ in range(n)]
		self.rev_graph = [[] for _ in range(n)]
		self.labels = [-1] * n
		self.lb_cnt = 0
	
	#グラフに有向辺を追加（逆向きのものを含む）
	def add_edge(self, v, nxt_v):
		self.graph[v].append(nxt_v)
		self.rev_graph[nxt_v].append(v)

	#SCCグラフの構成
	def build(self):
		self.post_order = []
		self.used = [False] * self.n
		for v in range(self.n):
			if(not self.used[v]):
				self._dfs(v)
		for v in reversed(self.post_order):
			if(self.labels[v] == -1):
				self._rev_dfs(v)
				self.lb_cnt += 1

	#SCCグラフを作る上でのDFS
	def _dfs(self, v):
		stack = [v, 0]
		while(stack):
			v, idx = stack[-2:]
			if not idx and self.used[v]:
				stack.pop()
				stack.pop()
			else:
				self.used[v] = True
				if idx < len(self.graph[v]):
					stack[-1] += 1
					stack.append(self.graph[v][idx])
					stack.append(0)
				else:
					stack.pop()
					self.post_order.append(stack.pop())

	#逆向きDFS
	def _rev_dfs(self, v):
		stack = [v]
		self.labels[v] = self.lb_cnt
		while stack:
			v = stack.pop()
			for nxt_v in self.rev_graph[v]:
				if self.labels[nxt_v] != -1:
					continue
				stack.append(nxt_v)
				self.labels[nxt_v] = self.lb_cnt

	#構成し終えたSCCグラフを連結成分ごとにリストにする
	def construct(self):
		self.dag = [[] for i in range(self.lb_cnt)]
		self.groups = [[] for i in range(self.lb_cnt)]
		for v, lb in enumerate(self.labels):
			for nxt_v in self.graph[v]:
				nxt_lb = self.labels[nxt_v]
				if lb == nxt_lb:
					continue
				self.dag[lb].append(nxt_lb)
			self.groups[lb].append(v)
		return self.dag, self.groups

"""
Main Code
"""

n,m = getNs()
route = [[] for _ in [0] * n]
scc = SCC(n)
for _ in [0] * m:
	a,b = [i - 1 for i in getNs()]
	route[a].append(b)
	scc.add_edge(a,b)
scc.build()
dag, _ = scc.construct()

s = len(dag)
if(s == 1):
	print(0)
	exit(0)

source = [True] * s
sink = [True] * s
for i in range(s):
	for j in dag[i]:
		sink[i] = source[j] = False
print(max(sum(source), sum(sink)))