/*
ID: hepic
PROG: sabotage
LANG: C++11
*/
#include <bits/stdc++.h>

#define FOR(i, a, b) for(auto i=a; i<=b; ++i)
#define REP(i, a, b) for(auto i=a; i<b; ++i)
#define FORI(i, a, b) for(auto i=a; i!=b+1-2*(a>b); i+=1-2*(a>b))
#define REPI(i, a, b) for(auto i=a-(a>b); i!=b-(a>b); i+=1-2*(a>b))
#define ALL(v) v.begin(),v.end()
#define mp(a, b) make_pair(a, b)
#define pb(a) push_back(a)
#define pf(a) push_front(a)
#define eb(a, b) emplace_back(a, b)
#define fir first
#define sec second
#define what_is(x) cout<<#x<<" is "<<x<<endl;
#define type(x) typeid(x).name()
#define ms(arr, val) memset(arr, val, sizeof(arr))
#define min3(a,b,c) min(min(a,b),c)
#define max3(a,b,c) max(max(a,b),c)
#define PI acos(-1)
#define open_read freopen("disrupt.in", "r", stdin)
#define open_write freopen("disrupt.out", "w", stdout)

using namespace std;

typedef long long LL;
typedef long double LD;
typedef unsigned long long ULL;
typedef pair<double, double> PDD;
typedef pair<int, int> PII;
typedef pair<int, PII> PIPII;
typedef pair<PII, PII> PPIIPII;
typedef pair<LL, LL> PLL;
typedef pair<ULL, ULL> PUU;
typedef pair<LL, PLL> PLPLL;
typedef pair<PLL, PLL> PPLLPLL;
typedef pair<int, LL> PIL;
typedef pair<LL, int> PLI;


template<typename T, typename T1>
ostream& operator << (ostream &out, pair<T, T1> obj)
{
    out << "(" << obj.first << ", " << obj.second << ")";
    return out;
}


template<typename T, typename T1>
ostream& operator << (ostream &out, map<T, T1> cont)
{
    typename map<T, T1>::const_iterator itr = cont.begin();
    typename map<T, T1>::const_iterator ends = cont.end();

    for(; itr!=ends; ++itr)
        out<<*itr<<" ";
    out<<endl;

    return out;
}


template<typename T>
ostream& operator << (ostream &out, set<T> cont)
{
    typename set<T>::const_iterator itr = cont.begin();
    typename set<T>::const_iterator ends = cont.end();

    for(; itr!=ends; ++itr)
        out<<*itr<<" ";
    out<<endl;

    return out;
}


template<typename T>
ostream& operator << (ostream &out, multiset<T> cont)
{
    typename multiset<T>::const_iterator itr = cont.begin();
    typename multiset<T>::const_iterator ends = cont.end();

    for(; itr!=ends; ++itr)
        out<<*itr<<" ";
    out<<endl;

    return out;
}


template<typename T, template<typename ELEM, typename ALLOC=allocator<ELEM>> class CONT>
ostream& operator << (ostream &out, CONT<T> cont)
{
    typename CONT<T>::const_iterator itr = cont.begin();
    typename CONT<T>::const_iterator ends = cont.end();

    for(; itr!=ends; ++itr)
        out<<*itr<<" ";
    out<<endl;

    return out;
}


template<typename T, unsigned int N, typename CTy, typename CTr>
typename enable_if<!is_same<T, char>::value, basic_ostream<CTy, CTr> &>::type
operator << (basic_ostream<CTy, CTr> &out, const T (&arr)[N])
{
     REP(i, 0, N)
        out<<arr[i]<<" ";
    out<<endl;

    return out;
}


template<typename T>
T gcd(T a, T b)
{
    T min_v = min(a, b);
    T max_v = max(a, b);

    while(min_v)
    {
        T temp = max_v % min_v;
        max_v = min_v;
        min_v = temp;
    }

    return max_v;
}


template<typename T>
T lcm(T a, T b)
{
    return (a*b) / gcd(a, b);
}


template<typename T>
T fast_exp_pow(T base, T exp, T mod)
{
    T res = 1;

    while(exp)
    {
        if(exp&1)
        {
            res *= base;
            res %= mod;
        }

        exp >>= 1;
        base *= base;
        base %= mod;
    }

    return res % mod;
}

/*#################################################################################################################
###################################################################################################################
###################################################################################################################
#################################################################################################################*/

#define SIZE 100010

int N, M, A, B, init;
int dpMax[SIZE], inDeg[SIZE];
vector<int> graph[SIZE], order;
stack<int> stc;


void topSort() {
    while (!stc.empty()) {
        int node = stc.top();
        stc.pop();

        order.pb(node);

        for (auto connNode: graph[node]) {
            if (--inDeg[connNode] == 0) {
                stc.push(connNode);
            }
        }
    }
}


int main() {
    scanf("%d%d", &N, &M);

    REP (i, 0, M) {
        scanf("%d%d", &A, &B);
        
        graph[A].pb(B);
        ++inDeg[B];
    }

    FOR (i, 1, N) {
        if (inDeg[i] == 0) {
            stc.push(i);
        }
    }
    
    topSort();
    int answer = 0;

    for (int i = order.size() - 1; i >= 0; --i) {
        for (auto j: graph[order[i]]) {
            dpMax[order[i]] = max(dpMax[order[i]], dpMax[j] + 1);
            answer = max(answer, dpMax[order[i]]);
        }
    }
    
    printf("%d\n", answer);
    return 0;
}


Measured memory_bytes: 7032