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

using namespace std;

const int N=100005;

int dp[N],vis[N];

vector<int>e[N],p;

void dfs(int v)

{

	vis[v]=1;

	for(int i=0;i<e[v].size();i++)	

		if(!vis[e[v][i]])

			dfs(e[v][i]);

	p.push_back(v);

}

int main()

{

	int n,m;

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

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

	{

		int x,y;

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

		e[x].push_back(y);

	}

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

		if(!vis[i])

			dfs(i);

	reverse(p.begin(),p.end());

	int ans=0;

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

	{

		int v=p[i];

		for(int j=0;j<e[v].size();j++)

			dp[e[v][j]]=max(dp[e[v][j]],dp[p[i]]+1);

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

	}

	printf("%d\n",ans);

	return 0;

}


// Faster C++ solution for the same problem (measured speedup: 2.498187538357894x):
#include<bits/stdc++.h>

#include <ext/pb_ds/tree_policy.hpp>

#include <ext/pb_ds/assoc_container.hpp>



using namespace __gnu_pbds;

using namespace std;





template <typename T>using ordered_set = tree<T, null_type, less<T>,rb_tree_tag, tree_order_statistics_node_update>;



#define PI              acos(-1.0)

#define en '\n'

#define Infi 0x3f3f3f3f

#define LInfi 0x3f3f3f3f3f3f3f3f



#define setzero(a)    memset(a,0,sizeof(a))

#define setneg(a)     memset(a,-1,sizeof(a))

#define setinf(a)     memset(a,126,sizeof(a))





#define READ_IN freopen("Input.txt","r",stdin);

#define READ_OUT freopen("Output.txt","w",stdout);





const int dx[] = {+1,-1,+0,+0};                         // 4 X directions

const int dy[] = {+0,+0,+1,-1};                         // 4 Y directions

const int dx8[] = {+0,+0,+1,-1,-1,+1,-1,+1};            // King X Moves

const int dy8[] = {-1,+1,+0,+0,+1,+1,-1,-1};            // King Y Moves

const int kx[] = {-2, -2, -1, -1,  1,  1,  2,  2};      // Knight X Moves

const int ky[] = {-1,  1, -2,  2, -2,  2, -1,  1};      // Knight Y Moves







int  Set    (int N,int pos) {return N=N | (1<<pos);}    /// turn a bit on

int  reset  (int N,int pos) {return N= N & ~(1<<pos);}  /// turn a bit off

bool check  (int N,int pos) {return (bool)(N & (1<<pos));} /// check, if a bit is on or not



void __print(int x) {cerr << x;}

void __print(long x) {cerr << x;}

void __print(long long x) {cerr << x;}

void __print(unsigned x) {cerr << x;}

void __print(unsigned long x) {cerr << x;}

void __print(unsigned long long x) {cerr << x;}

void __print(float x) {cerr << x;}

void __print(double x) {cerr << x;}

void __print(long double x) {cerr << x;}

void __print(char x) {cerr << '\'' << x << '\'';}

void __print(const char *x) {cerr << '\"' << x << '\"';}

void __print(const string &x) {cerr << '\"' << x << '\"';}

void __print(bool x) {cerr << (x ? "true" : "false");}



template<typename T, typename V>

void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ','; __print(x.second); cerr << '}';}

template<typename T>

void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? "," : ""), __print(i); cerr << "}";}

void _print() {cerr << "]\n";}

template <typename T, typename... V>

void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}

#ifndef ONLINE_JUDGE

#define trace(x...) cerr << "[" << #x << "] = ["; _print(x)

#else

#define trace(x...)

#endif



typedef long long int  ll;

typedef unsigned long long int llu;



vector<vector<int>> graph;



int dp[100005];



int make(int id){



    if(dp[id]!=-1)return dp[id];



    int ans = 0;

    for(int i=0;i<graph[id].size();i++){

        ans = max(ans,1+make(graph[id][i]));

    }

    return dp[id] = ans;

}



int main(void){



    int n,m;

    cin >> n >> m;



    graph.assign(n+2,vector<int>());



    for(int i=0;i<m;i++){

        int a,b;

        cin >> a >> b;

        graph[a].push_back(b);

    }

    int ans = 0;



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



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

        ans = max(ans,make(i));

    }



    cout << ans << endl;



    return 0;

}