// Slower C++ solution:
/*

	Author: Luis Manuel D�az Bar�n (LUISMO)

	Problem:

	Online Judge:

	Idea:

*/

#include<bits/stdc++.h>

// Types

#define ll long long

#define ull unsigned long long

// IO

#define sf scanf

#define pf printf

#define mkp make_pair

#define fi first

#define se second

#define endl "\n"



using namespace std;



typedef pair<int, int> pii;

typedef pair<ll, ll> pll;

const ll inf = 1e16 + 3;

const int mod = 1e9 + 7;

const int lim = 2e5 + 2;



int N, M;

vector<int> adj[lim];



int dp[lim];

bool mark[lim];



int DFS(int idx)

{

    if(mark[idx])

        return dp[idx];

    // for each adjacent

    for(int i = 0; i < adj[idx].size(); i++)

    {

        int nxt = adj[idx][i];

        int aux = DFS(nxt);

        dp[idx] = max(dp[idx], 1 + aux);

    }

    mark[idx] = true;

    return dp[idx];

}



void solve()

{

    cin >> N >> M;

    // reading edges

    for (int i = 0; i < M; i++)

    {

        int xi, yi;

        cin >> xi >> yi;

        adj[xi].push_back(yi);

    }

    

    int answ = 0;



    for(int i = 1; i <= N; i++)

    {

        int aux = DFS(i);

        answ = max(answ, aux);

    }



    cout << answ;

}



void fastIO()

{

	cin.sync_with_stdio(false);

	cin.tie(0);

}



void IO()

{

	if(fopen("d:\\lmo.in","r") != NULL)

	{

		freopen("d:\\lmo.in","r",stdin);

	}

	else if(fopen("/media/Beijing/lmo.in","r") != NULL)

	{

		freopen("/media/Beijing/lmo.in", "r", stdin);

	}

}



int main()

{

	IO();



	fastIO();



	solve();

}


// Faster C++ solution for the same problem (measured speedup: 1.8496320188185669x):
#include<set>

#include<map>

#include<cmath>

#include<ctime>

#include<queue>

#include<stack>

#include<cstdio>

#include<string>

#include<vector>

#include<random>

#include<cstdlib>

#include<cstring>

#include<iomanip>

#include<iostream>

#include<algorithm>

#include<unordered_map>

#define endl "\n"

#define fi first

#define se second

#define db double

#define gcd __gcd

#define pb push_back

#define mp make_pair

#define lowbit(x) (x & (-x))

#define PII  pair<int, int> 

#define all(x) x.begin(), x.end()

#define debug(x) cout << #x << " = " << x << endl

#define rep(i, a, b) for(__typeof(b) i = a; i <= (b); i++)

#define Rep(i, a, b) for(__typeof(a) i = a; i >= (b); i--)

#define FAST ios::sync_with_stdio(false); cin.tie(0); cout.tie(0)

template<class T> inline T qmin(T a, T b) { return a < b ? a : b; }

template<class T> inline T qmax(T a, T b) { return a > b ? a : b; }

typedef long long ll;

typedef unsigned long long ull;

const db eps = 1e-9;

const db PI = acos(-1);

const int inf = 0x3f3f3f3f;

const int mod = (int)1e9 + 7;

const int maxn = (int)1e5 + 5;//remember to modify it, No RE or MLE

const ll INF = 0x3f3f3f3f3f3f3f3f;

using namespace std;

ll qpow(ll x, ll y) { ll ret = 1; for( ; y; y >>= 1, x = (x * x) % mod) if(y & 1) ret = (ret * x) % mod; return ret; }



inline int read(){

	char ch = getchar();

	int x = 0, f = 1;

	while(ch < '0' || ch > '9'){

		if(ch == '-') f = -1;

		ch = getchar();

	}

	while('0' <= ch && ch <= '9'){

		x = x * 10 + ch - '0';

		ch = getchar();

	}

	return x * f;

}



int n, m;

vector<int> G[maxn];

int dp[maxn], ind[maxn];



void dfs(int u){

	for(int v : G[u]){

		if(!dp[v]){

			dfs(v);

			dp[u] = qmax(dp[u], dp[v] + 1);

		} else {

			dp[u] = qmax(dp[u], dp[v] + 1);

		}

	}

}



int main()

{

	n = read(), m = read();

	rep(i, 1, m) {

		int u = read(), v = read();

		G[u].pb(v);

		ind[v]++;

	}

	rep(i, 1, n){

		if(!ind[i]){

			dfs(i);

		}

	}

	printf("%d\n", *max_element(dp, dp + n + 1));

	return 0;

}