#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef unsigned int uint;
typedef unsigned char uchar;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;

#define REP(i,x) for(int i=0;i<(int)(x);i++)
#define REPS(i,x) for(int i=1;i<=(int)(x);i++)
#define RREP(i,x) for(int i=((int)(x)-1);i>=0;i--)
#define RREPS(i,x) for(int i=((int)(x));i>0;i--)
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();i++)
#define RFOR(i,c) for(__typeof((c).rbegin())i=(c).rbegin();i!=(c).rend();i++)
#define ALL(container) (container).begin(), (container).end()
#define RALL(container) (container).rbegin(), (container).rend()
#define SZ(container) ((int)container.size())
#define mp(a,b) make_pair(a, b)
#define pb push_back
#define eb emplace_back
#define UNIQUE(v) v.erase( unique(v.begin(), v.end()), v.end() );
#define __builtin_popcount __builtin_popcountll

template<class T> bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }
template<class T> ostream& operator<<(ostream &os, const vector<T> &t) {
    os<<"["; FOR(it,t) {if(it!=t.begin()) os<<","; os<<*it;} os<<"]"; return os;
}
template<class T> ostream& operator<<(ostream &os, const set<T> &t) {
    os<<"{"; FOR(it,t) {if(it!=t.begin()) os<<","; os<<*it;} os<<"}"; return os;
}
template<class S, class T> ostream& operator<<(ostream &os, const map<S, T> &t) {
    os<<"{"; FOR(it,t) {if(it!=t.begin()) os<<","; os<<*it;} os<<"}"; return os;
}
template<class S, class T> ostream& operator<<(ostream &os, const pair<S,T> &t) { return os<<"("<<t.first<<","<<t.second<<")";}
template<class S, class T> pair<S,T> operator+(const pair<S,T> &s, const pair<S,T> &t){ return pair<S,T>(s.first+t.first, s.second+t.second);}
template<class S, class T> pair<S,T> operator-(const pair<S,T> &s, const pair<S,T> &t){ return pair<S,T>(s.first-t.first, s.second-t.second);}

const int INF = 1<<28;
const double EPS = 1e-8;
const int MOD = 1000000007;

struct UnionFind {
    vector<int> d;
    UnionFind(int n=0) : d(n, -1){}
    bool unite(int x, int y) {
        if ((x = root(x)) != (y = root(y))){
            if (d[y] < d[x]) swap(x, y);
            d[x] += d[y]; d[y] = x;
        }
        return x != y;
    }
    bool find(int x, int y){return root(x) == root(y);}
    int root(int x){return d[x] < 0 ? x : d[x] = root(d[x]);}
    int size(int x=-1) {return x<0 ? d.size() : -d[root(x)];}
};


struct SCC{
    int n;
    vector<vi> g, rg, ng;
    vector<vi> comps;
    UnionFind uf;
    SCC(const vector<vi> &g):n(g.size()),g(g),rg(n),ng(n),comps(n),uf(n){
        REP(i, n)REP(j, g[i].size()) rg[g[i][j]].pb(i);
        vi d, vis(n);
        REP(i, n) if(!vis[i]){
                stack<int> st;
                st.push(i);
                while(!st.empty()){
                    int r = st.top(); st.pop();
                    if(r >= 0){
                        if(vis[r]) continue;
                        vis[r] = 1;
                        st.push(~r);
                        for(int v : g[r]) if(!vis[v]) st.push(v);
                    }else d.push_back(~r);
                }
            }
        vis = vi(n);
        RREP(i, n) if(!vis[d[i]]){
                stack<int> st;
                st.push(d[i]);
                while(!st.empty()){
                    int r = st.top(); st.pop();
                    if(vis[r]) continue;
                    vis[r] = 1;
                    for(int v : rg[r]) if(!vis[v]){
                            uf.unite(r, v);
                            st.push(v);
                        }
                }
            }
        REP(i, n) for(int v : g[i]) if(uf.root(i) != uf.root(v)) ng[uf.root(i)].push_back(uf.root(v));
        REP(i, n) sort(ALL(ng[i])), UNIQUE(ng[i]);
        REP(i, n) comps[uf.root(i)].pb(i);
    }
    vi TopologicalSort(){
        vi deg(n), res;
        REP(i, n)REP(j, ng[i].size()) deg[ng[i][j]] ++;
        queue<int> q;
        REP(i, n)if(uf.root(i) == i && deg[i] == 0) q.push(i);
        while(!q.empty()){
            int u = q.front(); q.pop();
            res.pb(u);
            for(int v : ng[u])if(--deg[v] == 0)	q.push(v);
        }
        return res;
    }
};

int T, n, m;

int main(int argc, char *argv[]){
    ios::sync_with_stdio(false);
    while(cin >> n >> m){
        vector<vi> g(n);
        REP(i, m) {
            int u, v;
            cin >> u >> v; u--; v--;
            g[u].push_back(v);
        }
        SCC scc(g);
        vi dp(n);
        for(int u : scc.TopologicalSort()) {
            for (int v: scc.ng[u]) {
                chmax(dp[v], dp[u] + 1);
            }
        }
        cout << *max_element(ALL(dp)) << endl;
    }
    return 0;
}


Measured memory_bytes: 29736