// Slower C++ solution:
#include<bits/stdc++.h>

using namespace std;

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

#define uniq(x) (x).erase(unique(ALL(x)),(x).end())

#define SORT(x) sort(ALL(x))

#define REV(x) reverse(ALL(x))

#define foreach(it,x) for(__typeof((x).begin()) it=(x).begin();it!=(x).end();it++)

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

#define gett() cerr<<"Time:"<<clock()<<"ms."<<endl

#define ri(x) scanf("%d",&x)

#define rl(x) scanf("%lld",&x)

#define rlcfpoj(x) scanf("%I64d",&x)

#define pi(x) printf("%d",x)

#define pl(x) printf("%lld",x)

#define plcfpoj(x) printf("%I64d",x)

#define lfd() printf("\n")

#define sp() printf(" ")

#define inrep(x) int x;scanf("%d",&x);while(x--)

#define testbit(mask,i) (((mask)>>(i))&1)

#define setbit(mask,i) ((mask)|(1<<i))

#define delbit(mask,i) ((mask)^(1<<i))

int dp[100100];

int n,m;

vector<int> ga[100100];

int u,v;

int d(int x){

	if(dp[x]!=-1)

		return dp[x];

	int &res=dp[x];

	res=0;

	for(int i=0;i<ga[x].size();i++){

		int y=ga[x][i];

		res=max(res,d(y)+1);

	}

	return res;

}

int main(){

	//freopen("input.txt","r",stdin);

	//freopen("output.txt","w",stdout);

	ios::sync_with_stdio(false);

	cin.tie(NULL);

	cout.tie(NULL);

	cin>>n>>m;

	for(int i=1;i<=m;i++){

		cin>>u>>v;

		ga[u].push_back(v);

	}

	int res=0;

	memset(dp,-1,sizeof(dp));

	for(int i=1;i<=n;i++){

		res=max(res,d(i));

	}

	cout<<res<<endl;

	return 0;

}


// Faster C++ solution for the same problem (measured speedup: 1.1196241969072234x):
#include <bits/stdc++.h> 



using namespace std;

 

typedef long long ll;

typedef long double ld;

typedef unsigned long long ull;

 

#define nl '\n'

 

#define y1 aza_ibagaliev

 

#define f first

#define s second

	

#define ed end()

#define bg begin()

#define rbg rbegin()

 

#define rv reverse

#define mp make_pair

#define pb push_back

#define ppb pop_back

#define ppf pop_front

 

#define sz size()

#define all(x) x.bg, x.ed

 

#define pll pair < ll, ll >

#define pii pair < int, int >

 

#define sqr(x) ((x) * 1ll * (x))

#define sqrd(x) ((x) * 1.0 * (x))

 

#define speed ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0)

 

#define rnl cout << "----------------------------------------" << nl

 

#define bit __builtin_popcount

 

inline ll bip(ll x, ll n, ll mod){ll res=1;while(n){if(n & 1){res=(res*x)%mod;}x=(x*x)%mod;n>>=1;}return res;}

        	

const int ppr = 257;

const ll INF = 2e18;

const int inf = 2e9;

const int mod = 1e9 + 7;

const int N = 1e5 + 123;

const ld pi = 3.1741592653589793238462643;

 

const int dx[] = {1, 0, -1, 0};

const int dy[] = {0, 1, 0, -1};



int n, m, dp[N], ans, cnt[N], gr[N];

vector<int>g[N],ord;

bool u[N];



void top(int v) {

	u[v] = 1;

	for(int to : g[v]) 

		if(!u[to]) top(to);

	ord.pb(v);

}



void dfs(int v) {

	u[v] = 1;

	for(int to : g[v])

		if(!u[to]) dfs(to);

}



main() {             	

//  freopen(".in", "r", stdin);

//  freopen(".out", "w", stdout);



//	srand(time(NULL));



//	speed;



	scanf("%d%d", &n, &m);



	while(m--) {

		int x, y;

		scanf("%d%d", &x, &y);

		g[x].pb(y);

		gr[y]++;

	}



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

		if(!u[i]) top(i);

            

	rv(all(ord));



	queue<int>q;



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

		if(!gr[i]) q.push(i);



	while(!q.empty()) {

		int v = q.front();

		q.pop();



		ans = max(ans, dp[v]);



		for(int to : g[v]) {

			dp[to] = max(dp[to], dp[v] + 1);

			cnt[to]++;

			if(cnt[to] == gr[to]) q.push(to);

		}

	}



	cout << ans;



    return 0;                                        

}