#include<bits/stdc++.h>

#define Input                   freopen("in.txt","r",stdin)
#define Output                  freopen("out.txt","w",stdout)
#define ll                      long long int
#define ull                     unsigned long long int
#define pii                     pair<int,int>
#define pll                     pair<ll,ll>
#define sc                      scanf
#define scin(x)                 sc("%d",&(x))
#define scin2(x,y)                sc("%d %d",&(x),&(y))
#define scln(x)                 sc("%lld",&(x))
#define scln2(x,y)                sc("%lld %lld",&(x),&(y))
#define pf                      printf
#define ms(a,b)                 memset(a,b,sizeof(a))
#define pb(a)                   push_back(a)
#define mp                      make_pair
#define db                      double
#define EPS                     10E-10
#define ff                      first
#define ss                      second
#define sqr(x)                  (x)*(x)
#define vi                      vector<int>
#define vl                      vector<ll>
#define vii                     vector<vector<int> >
#define vll                     vector<vector<ll> >
#define DBG                     pf("HI\n")
#define MOD                     1000000007
#define CIN                     ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0)
#define RUN_CASE(t,T)           for(__typeof(t) t=1;t<=T;t++)
#define CASE(t)                 printf("Case %d: ",t)
#define CASEl(t)                printf("Case %d:\n",t)
#define intlimit                214748369
#define longlimit               92233720368547758
#define infinity                (1<<28)
#define gcd(a,b)                __gcd(a,b)
#define lcm(a,b)                ((a)*(b))/gcd(a,b)
#define mxx                     123456789
#define PI                      2*acos(0.0)
#define rep(i,a,b)              for(__typeof(i) i=a;i<=b;i++)
#define rev(i,a,b)              for(__typeof(i) i=a;i>=b;i--)

using namespace std;
/** toint, tostring, BigMod, Power , sieve, Primefactorize ,frequency in n!**/
//ll toint(string s){ll n=0,k=1;for(int i=s.size()-1;i>=0;i--){n += ((s[i]-'0')*k);k*=10;}return n;}
//ll BigMod(ll a,ll b,ll m){if(b == 0)return 1%m;else if(b%2 == 0){ll temp=BigMod(a,b/2,m);return ((temp%m)*(temp%m))%m;}else{return ((a%m)*BigMod(a,b-1,m)%m)%m;}}
//ll frequency(ll n,ll factor)/** Frequency of a factor in n! **/{ll cnt=0;while(n){cnt += (n/factor);n /= factor;}return cnt;}

/** Order Set **/
//#include <ext/pb_ds/assoc_container.hpp>
//using namespace __gnu_pbds;
//template<typename T> using orderset = tree<T,null_type,less<T>,rb_tree_tag,tree_order_statistics_node_update>;
//orderset<int> s ; //orderset<int>::iterator it ;
//orderset<int> X; //X.insert(1); //X.insert(2); //X.insert(4); //X.insert(8); //X.insert(16);
//cout<<*X.find_by_order(1)<<endl; // 2 //cout<<*X.find_by_order(2)<<endl; // 4 //cout<<*X.find_by_order(4)<<endl; // 16 //cout<<(end(X)==X.find_by_order(6))<<endl; // true
//cout<<X.order_of_key(-5)<<endl;  // 0 //cout<<X.order_of_key(1)<<endl;   // 0 //cout<<X.order_of_key(3)<<endl;   // 2 //cout<<X.order_of_key(4)<<endl;   // 2 //cout<<X.order_of_key(400)<<endl; // 5

int n,m;
vi graph[100005];
bool vis[100005];
int dp[100005];
int fun(int start)
{
    if(graph[start].size() == 0){
//        pf("%d have length zero.\n",start);
        return 0;
    }
    if(dp[start] != -1)
        return dp[start];
    int ret1=0,opt=0;
    for(int i=0;i<graph[start].size();i++)
    {
        ret1 = 1+fun(graph[start][i]);
        opt=max(opt,ret1);
    }
//    pf("length from %d = %d\n",start,opt);
    return dp[start]=opt;
}
int main()
{
    int i,j,k,u,v,ans=0,x;
    ms(dp,-1);
    ms(vis,0);
    scin2(n,m);
    for(i=1;i<=m;i++)
    {
        scin2(u,v);
        graph[u].pb(v);
    }
    for(i=1;i<=n;i++)
    {
        x=fun(i);
        ans=max(ans,x);
    }
    pf("%d\n",ans);
    return 0;
}



Measured memory_bytes: 8192