#include<iostream>

 #include<cstdio>

 #include<cstring>

 #include<algorithm>

 #include<cmath>

 #include<queue>

 #define dbg(x) cerr << #x << " = " << x <<endl

 #define dbg2(x,y) cerr<< #x <<" = "<< x <<"  "<< #y <<" = "<< y <<endl

 using namespace std;

 typedef long long ll;

 typedef double db;

 typedef pair<int,int> pii;

 template<typename T>inline T _min(T A,T B){return A<B?A:B;}

 template<typename T>inline T _max(T A,T B){return A>B?A:B;}

 template<typename T>inline char MIN(T&A,T B){return A>B?(A=B,):;}

 template<typename T>inline char MAX(T&A,T B){return A<B?(A=B,):;}

 template<typename T>inline void _swap(T&A,T&B){A^=B^=A^=B;}

 template<typename T>inline T read(T&x){

     x=;int f=;char c;while(!isdigit(c=getchar()))if(c=='-')f=;

     while(isdigit(c))x=x*+(c&),c=getchar();return f?x=-x:x;

 }

 const int N=1e5+,INF=0x3f3f3f3f;

 struct thxorz{int to,nxt;}G[N],dag[N],rdag[N];

 int Head[N],dhd[N],rhd[N],tot1,tot2,tot3,frm[N];

 int n,m;

 inline void Addedge(int x,int y){G[++tot1].to=y,G[tot1].nxt=Head[x],Head[x]=tot1;frm[tot1]=x;}

 inline void AddDAG(int x,int y){dag[++tot2].to=y,dag[tot2].nxt=dhd[x],dhd[x]=tot2;}

 inline void AddrDAG(int x,int y){rdag[++tot3].to=y,rdag[tot3].nxt=rhd[x],rhd[x]=tot3;}

 #define y G[j].to

 int dfn[N],low[N],cnt,instk[N],stk[N],Top,val[N],bel[N];

 void tarjan(int x){

     dfn[x]=low[x]=++cnt,stk[++Top]=x,instk[x]=;

     for(register int j=Head[x];j;j=G[j].nxt){

         if(!dfn[y])tarjan(y),MIN(low[x],low[y]);

         else if(instk[y])MIN(low[x],dfn[y]);

     }

     if(dfn[x]==low[x]){int tmp;do instk[tmp=stk[Top--]]=,bel[tmp]=x,++val[x];while(tmp^x);}

 }

 #undef y

 int ans1[N],ans2[N],vis[N],ans;

 #define y rdag[j].to

 int dp1(int x){//dbg2(x,val[x]);

     if(vis[x])return ans1[x];

     vis[x]=;

     for(register int j=rhd[x];j;j=rdag[j].nxt)MAX(ans1[x],dp1(y)+val[x]);

     return ans1[x];

 }

 #undef y

 #define y dag[j].to

 int dp2(int x){

     if(vis[x])return ans2[x];

     vis[x]=;

     for(register int j=dhd[x];j;j=dag[j].nxt)MAX(ans2[x],dp2(y)+val[x]);

     return ans2[x];

 }

 #undef y

 int main(){//freopen("test.in","r",stdin);//freopen("test.ans","w",stdout);

     read(n),read(m);

     for(register int i=,x,y;i<=m;++i)read(x),read(y),Addedge(x,y);

     for(register int i=;i<=n;++i)if(!dfn[i])tarjan(i);

     for(register int t=,u,v;t<=tot1;++t){

         u=frm[t],v=G[t].to;

         if(bel[u]^bel[v])AddDAG(bel[u],bel[v]),AddrDAG(bel[v],bel[u]);//CAUTION!LOOP!

     }

     fill(ans1+,ans1+n+,-INF),fill(ans2+,ans2+n+,-INF);ans1[bel[]]=ans2[bel[]]=;

     for(register int i=;i<=n;++i)if(val[i])dp1(i);

     memset(vis,,sizeof vis);ans=val[bel[]];

     for(register int i=;i<=n;++i)if(val[i])dp2(i);

     for(register int t=,u,v;t<=tot2;++t){

         u=rdag[t].to,v=dag[t].to;

         MAX(ans,ans1[v]+ans2[u]+val[bel[]]);

     }

     printf("%d\n",ans);

     return ;

 }