#強連結成分分解
#分解したら、sG を　頂点数num のグラフとして普通に扱えばいい
#sG は set で作ってある
#Componentに各成分を構成するもとの頂点を入れてる
class SCC():

    #index :0-index or 1-index
    #逆グラフは自動で作る
    def __init__(self,G,index = 1):
        self.G = G
        self.N = len(G)
        self.index = index
        self.rG = self.make_reverse()
        self.order = []
        self.parent = [0] * self.N  #各頂点の属する強連結成分の番号

        self.num = 0     #強連結成分の数
        
        self.build()
        self.sG,self.top,self.Component = self.make_sG()
        
        #sG 縮約後のグラフ。setでつくってある
        #top sGをトポロジカルソートしたもの
        #Component 各強連結成分に属する頂点をまとめた配列

    #逆グラフを作る
    def make_reverse(self):
        G = self.G
        rG = [[] for _ in range(self.N)]
        for v in range(self.index,self.N):
            for u in G[v]:
                rG[u].append(v)
        return rG
    def build(self):
        G = self.G
        rG = self.rG
        parent = self.parent
        order = self.order

        memo = [0] * self.N
        stack = []
        label = 0
        for v in range(self.index,self.N):
            if memo[v] == 1:continue
            stack.append(~v)
            stack.append(v)
            memo[v] = 1
            while stack:
                now = stack.pop()
                if now >= 0:
                    for u in G[now]:
                        if memo[u] == 0:
                            memo[u] = 1
                            stack.append(~u)
                            stack.append(u)
                else:
                    order.append(~now)
        memo = [0] * self.N
        for v in reversed(order):
            if memo[v] == 1:continue
            stack.append(v)
            memo[v] = 1
            parent[v] = label
            while stack:
                now = stack.pop()
                for u in rG[now]:
                    if memo[u] == 1:
                        continue
                    memo[u] = 1
                    stack.append(u)
                    parent[u] = label
            label += 1
        self.num = label
    #縮約後のグラフを作る
    #トポロジカルソートもする
    def make_sG(self):
        sG = [set() for _ in range(self.num)]
        sGnum = [0] * self.num
        Component = [[] for _ in range(self.num)]
        G = self.G
        parent = self.parent
        for v in range(self.index,self.N):
            Component[parent[v]].append(v)
            for u in G[v]:
                if parent[u] == parent[v]:continue
                if parent[u] not in sG[parent[v]]:
                    sG[parent[v]].add(parent[u])
                    sGnum[parent[u]] += 1
        top = []
        stack = []
        for i in range(self.num):
            if sGnum[i] == 0:
                stack.append(i)
        while stack:
            now = stack.pop()
            top.append(now)
            for v in sG[now]:
                sGnum[v] -= 1
                if sGnum[v] == 0:
                    stack.append(v)
        return sG,top,Component
        
N,M = map(int,input().split())
G = [[] for _ in range(N + 1)]
for _ in range(M):
    a,b = map(int,input().split())
    G[b].append(a)
scc = SCC(G,1)
sG = scc.sG
n = scc.num
inGnum = [0] * n
outGnum = [0] * n
for i in range(n):
    for v in sG[i]:
        inGnum[v] += 1
        outGnum[i] += 1
import sys
if n == 1:
    print(0)
    exit()
_in = 0
out = 0
for i in range(n):
    if inGnum[i] == 0:
        _in += 1
    if outGnum[i] == 0:
        out += 1
print(max(_in,out))