#include <iostream>
#include <iomanip>
#include <fstream>
#include <string>
#include <array>
#include <vector>
#include <deque>
#include <list>
#include <set>
#include <map>
#include <unordered_map>
#include <unordered_set>
#include <stack>
#include <queue>
#include <bitset>
#include <tuple>
#include <cmath>
#include <complex>
#include <algorithm>
#include <utility>
#include <regex>
#include <cstdint>
#include <numeric>
#include <functional>
#include <cassert>

using namespace std;
using ll = int64_t;
using ull = uint64_t;

constexpr ll INF = 1000000000;/* 1e+9a */
namespace utils{

    class Repeater{
        int st;
        int ed;
        int d;
    public:
        class rep_iterator{
            int i; int d;
        public:
            rep_iterator(int x, int pd) :i(x), d(pd) {}

            int& operator*(){ return i; }
            bool operator==(const rep_iterator& k) const { return i == k.i; }
            bool operator!=(const rep_iterator& k) const { return i != k.i; }
            rep_iterator& operator++(){ i+=d; return *this; }
        };

        explicit Repeater(int N) :st(0), ed(N), d(1) {}
        Repeater(int b, int e) :st(b), ed(e), d(1) {if(st > ed){ st--; ed--; d = -1; } }

        rep_iterator begin() const{ return {st, d}; }
        rep_iterator end() const{ return {ed, d}; }

    };

    template<class T, class Compare> using p_queue = priority_queue<T, vector<T>, Compare>;
    template<class T> using min_queue = p_queue<T, greater<T>>;
    template<class T> using max_queue = p_queue<T, less<T>>;

    template<class T> bool min_update(T& X, const T& A){ if(X > A) {X = A; return true;} return false; }
    template<class T> bool max_update(T& X, const T& A){ if(X < A) {X = A; return true;} return false; }
#define ALL(x) begin(x), end(x)
#define rALL(x) rbegin(x), rend(x)
}
using namespace utils;

using V_Set = unordered_set<int>;
using E_Set = unordered_map<int, V_Set>;

vector<int> toporogical_sort(const E_Set& edge, int n){
    unordered_map<int, int> in_deg;
    for (auto &&E : edge) {
        for (auto &&e : E.second) {
            in_deg[e]++;
        } // end e
    } // end e

    stack<int> stk;
    for (int j = 0; j < n; ++j) {
        if(in_deg[j] == 0)
            stk.push(j);
    } // end j

    vector<int> tps;
    while(tps.size() < n){
        int t = stk.top(); stk.pop();
        tps.push_back(t);

        if(edge.count(t) > 0)
            for (auto &&a : edge.at(t)) {
                in_deg[a]--;
                if(in_deg[a] == 0) stk.push(a);
            } // end a
    }

    return tps;
}

void sub_main(istream &is) {
    ll N, M;
    is >> N >> M;
    if(!is) return;

    E_Set edge, in_edge;
    for (int i = 0; i < M; ++i) {
        int x, y;
        is >> x >> y;
        x--; y--;
        edge[x].insert(y);
        in_edge[y].insert(x);
    } // end i
    auto tps = toporogical_sort(edge, N);
    vector<int> dp(N);

    for(auto &&v : tps) {
        dp[v] = 0;
        for (auto &&s : in_edge[v]) {
            max_update(dp[v], dp[s] + 1);
        } // end s
    } // end i

    int mx = 0;
    for (auto &&i : dp) {
        max_update(mx, i);
    } // end i

    cout << mx << endl;
}

int main(int argc, char *argv[]) {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << setprecision(16) << scientific;
#ifdef DEBUG
    string test_cases = "test_G.txt";
    cerr << "DEBUG MODE" <<  endl;
    cerr << test_cases << " -->" << endl;
    auto fs = fstream(test_cases, fstream::in);
    int lp = 0;
    while(fs) {
        lp++;
        cout << lp <<  "#------\n";
        sub_main(fs);
    }
    cout << "------#" << endl;
    if(lp <= 1) sub_main(cin);
#else
    sub_main(cin);
#endif
    return 0;
}


Measured memory_bytes: 35400