#include<stdio.h>#include<string.h>#include<algorithm>#include<iostream>#include<vector>#include<map>#include<stack>using namespace std;#define maxn 110000vector<int>old[maxn];vector<int>vec[maxn];int dnf[maxn],low[maxn],instack[maxn];int times,nums;stack<int>st;int pan[maxn];int ru[maxn];int vis[maxn];int ans;int res[maxn];int val[maxn];void init(int n){ for(int i=0;i<=n+1;i++) { dnf[i]=low[i]=instack[i] =pan[i]=ru[i]=vis[i]=res[i]=val[i]=0; vec[i].clear();old[i].clear(); } while(!st.empty())st.pop(); times=nums=1;}void tarjan(int x){ dnf[x]=low[x]=times++; instack[x]=1; st.push(x); for(int i=0;i<old[x].size();i++) { int y=old[x][i]; if(!dnf[y]) { tarjan(y); low[x]=min(low[x],low[y]); } else if(instack[y]) { low[x]=min(low[x],dnf[y]); } } if(low[x]==dnf[x]) { int y=-1; while(x!=y) { y=st.top(); st.pop(); instack[y]=0; pan[y]=nums; val[nums]++; } nums++; }}void dfs(int x){ if(res[x])return; res[x]=val[x]; for(int i=0;i<vec[x].size();i++) { int y=vec[x][i]; dfs(y); res[x]=max(res[x],res[y]+val[x]); }}int main(){ int n,m,x,y; while(~scanf("%d%d",&n,&m)) { init(n); while(m--) { scanf("%d%d",&x,&y); old[x].push_back(y); } for(int i=1;i<=n;i++) if(!dnf[i])tarjan(i); for(int i=1;i<=n;i++) { for(int j=0;j<old[i].size();j++) { x=pan[i]; y=pan[old[i][j]]; if(x==y)continue; vec[x].push_back(y); ru[y]++; } } ans=-1; for(int i=1;i<nums;i++) { if(!ru[i]) { dfs(i); ans=max(ans,res[i]); } } cout<<ans<<endl; } return 0;}
