/**
 * code generated by JHelper
 * More info: https://github.com/AlexeyDmitriev/JHelper
 * @author
 */

#include <iostream>
#include <fstream>

#include <iostream>
#include "bits/stdc++.h"

using namespace std;
typedef unsigned long long ull;
typedef long long ll;
typedef int                     integer;
#define int                     long long
typedef pair<int, int>          pii;
#define vii                     vector<pair<int,int>>
#define vi                      vector<int>
#define vvi                     vector<vector<int>>
#define rep(i, a, b)            for(int i = a; i < (b); i++)
#define s(x)                    in>>x
#define s2(x,y)                 in>>x>>y
#define s3(x,y,z)               in>>x>>y>>z
#define s4(x,y,z,w)             in>>x>>y>>z>>w
#define p(x)                    out<<(x)<<endl
#define p2(x,y)                 out<<(x)<<' '<<(y)<<endl
#define p3(x,y,z)               out<<(x)<<' '<<(y)<<' '<<(z)<<endl
#define p4(x,y,z,w)             out<<(x)<<' '<<(y)<<' '<<(z)<<' '<<(w)<<endl
#define p1d(a,n)                for(int ix=0;ix<n-1;ix++) out<<a[ix]<<' '; out<<a[n-1]<<'\n';
#define p2d(a,n,m)              for(int ix=0;ix<n;ix++){ for(int jx=0;jx<m;jx++) out<<a[ix][jx]<<" "; out<<endl;}
#define pl(x)                   out<<(x)
#define pll(x)                  out<<(x)<<' '
#define range(v)                v.begin(),v.end()
#define all(x)                  begin(x), end(x)
#define sz(x)                   (int)(x).size()
#define rst(x)                  memset(x, -1, sizeof(x))
#define UNVISITED               -1
#define VISITED                 0
#define EXPLORED                2

#define EACH(x, a) for (auto& x: a)
template<class T> void read_v(T& x, std::istream& in) {
    in >> (x);
}
template<class A> void read_vec(vector<A>& x, std::istream& in) {
    EACH(a, x)
        read_v(a, in);
}

ll iceildiv(const ll a, const ll b){ if(a==0){return 0;} return (1 + ((a - 1)/b));}
bool sortpairbysec(const pair<int,int> &a,const pair<int,int> &b){return (a.second < b.second);}
template <typename T>
T factmod(T a, T modulus){ int res = 1, i; for (i = 2; i <= a; i++){ res *= i; res%=modulus;} return res; }
template <typename T>
int powmod(T a, T b, const int m){ int ans=1; while(b>0){if(b&1){ans*=a;ans%=m;}a*=a;a%=m;b>>=1;} return ans; }
template <typename T>
int power(T a, T b){ int ans=1; while(b>0){if(b&1){ans*=a;}a*=a;b>>=1;}return ans;}

int dfs(int node, const vector<vector<int>>& edges, vector<int>& dp){
    if(dp[node] != 0){return dp[node];}
    int sz = 0;
    for(const int children: edges[node]){
        if(dp[children] == 0){
            int aux = dfs(children, edges, dp);
            sz = max(sz, aux);
        }else{
            sz = max(sz, dp[children]);
        }
    }
    dp[node] = 1 + sz;
    return dp[node];
}

void ssolve(std::istream& in, std::ostream& out){
    int n,m; s2(n, m);
    vvi edges(n + 1);
    rep(i, 0, m){
        int from, to; s2(from, to);
        edges[from].emplace_back(to);
    }
    vi dp(n + 1, 0);
    rep(i, 1, n + 1){
        dfs(i, edges, dp);
    }

    int ans = *max_element(all(dp));
    p(ans - 1);
    return;
}

class GLongestPath {
public:
void solve(std::istream& in, std::ostream& out) {
    ios_base::sync_with_stdio(false);
    in.tie(nullptr);out.tie(nullptr);
    int t;
    int READ = 0;
    if(READ) s(t);
    else t = 1;
    while(t--){
        ssolve(in, out);
    }
}
};

integer main() {
    GLongestPath solver;
    std::istream& in(std::cin);
     std::ostream& out(std::cout);
     solver.solve(in, out);
}

Measured memory_bytes: 12776