n,m=map(int,input().split())
e=[]
for i in range(m):
  a,b=map(int,input().split())
  e+=[(a-1,b-1)]

class SCC():
  def __init__(self,n):
    self.n=n
    self.e=[[] for i in range(self.n)]
    self.re=[[] for i in range(self.n)]
    return
  
  def add_edge(self,s,t):
    self.e[s]+=[t]
    self.re[t]+=[s]
    return
  
  def scc(self):
    v=[0]*self.n
    g=[0]*self.n
    o=[]
    for i in range(self.n):
      if v[i]==0:
        q=[i]
        while len(q)>0:
          s=q[-1]
          v[s]=1
          while g[s]<len(self.e[s]):
            t=self.e[s][g[s]]
            if v[t]==0:
              break
            g[s]+=1
          if g[s]<len(self.e[s]):
            q+=[t]
          else:
            o+=[s]
            q.pop()
    c=[-1]*self.n
    p=0
    for i in o[::-1]:
      if c[i]==-1:
        s=i
        c[s]=p
        q=[s]
        for s in q:
          for t in self.re[s]:
            if c[t]==-1:
              c[t]=p
              q+=[t]
        p+=1
    l=[[] for i in range(p)]
    for i in range(self.n):
      l[c[i]]+=[i]
    d=[0]*p
    for s in range(self.n):
      for t in self.e[s]:
        if c[s]!=c[t]:
          d[c[t]]+=1
    ts=[]
    q=[i for i in range(p) if d[i]==0]
    for v in q:
      ts+=[v]
      for s in l[v]:
        for t in self.e[s]:
          if c[s]!=c[t]:
            d[c[t]]-=1
            if d[c[t]]==0:
              q+=[c[t]]
    return [l[ts[i]] for i in range(p)]

g=SCC(n)
for a,b in e:
  g.add_edge(a,b)
L=g.scc()
l=len(L)
c=[0]*n
for i in range(l):
  for j in L[i]:
    c[j]=i
do=[0]*l
di=[0]*l
for a,b in e:
  if c[a]!=c[b]:
    do[c[a]]=1
    di[c[b]]=1

print(max(sum(do[i]==0 for i in range(l)),sum(di[i]==0 for i in range(l))))