// Slower C++ solution:


/**

    And in the end, the love you take is equal to the love you make.



        I LOVE YOU LAURA.

**/



#include <bits/stdc++.h>

#include <ext/pb_ds/assoc_container.hpp>

#include <ext/pb_ds/tree_policy.hpp>

#include <ext/rope>



#define _CRT_SECURE_NO_DEPRECATE

#define _CRT_DISABLE_PERFCRIT_LOCKS



#define findOrder find_by_order

#define orderKey order_of_key



#define isOn(mask, i) (mask & (1 << i))

#define turn(mask, i) (mask |= (1 << i))

#define switch(mask, i) (mask &= ~(1 << i))

#define change(mask, i) (mask ^= (1 << i))

#define lowbit(mask) (mask & (-mask))

#define turnAll(mask, n) (mask = (1 << n) - 1)

#define switchLastBit(mask) (mask & (mask - 1))

#define turnLastZero(mask) (mask | (mask + 1))

#define modulo(x, mod) ((x) & (mod - 1))

#define log2(x) (31 - __builtin_clz(x))



#define mkp make_pair

#define pb push_back

#define pf push_front

#define eb emplace_back

#define ef emplace_front

#define ins insert

#define rsz resize

#define f first

#define s second

#define lb lower_bound

#define ub upper_bound



#define sz(x) ((int)x.size())

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

#define rall(x) rbegin(x), rend(x)



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



using namespace std;

using namespace __gnu_pbds;

using namespace __gnu_cxx;



typedef double db;

typedef long long ll;

typedef long double ld;

typedef unsigned long long ull;



typedef pair<int, int> ii;

typedef pair<ll,ll> pll;

typedef pair<ld,ld> pdb;

typedef pair<int, ii> tri;

typedef complex<ld> cd;



typedef vector<int> vi;

typedef vector<ii> vii;

typedef vector<ld> vdb;

typedef vector<cd> vcd;

typedef vector<ll> vll;

typedef vector<string> vs;

typedef vector<vll> matrix;



typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> indexed_tree;



const ll mod = 1e9+7;

const int maxn = 1e5+2;

const int pi = 2 * acos(0.0);

const db eps = 1e-9;



ll n, m;

vll graph[maxn];

ll dp[maxn];



ll dfs (int node){

    ll &ans = dp[node];



    if (ans != -1) return ans;



    for (auto v: graph[node]){

        ans = max(ans, dfs(v) + 1);

    }

    return ans;

}



int main()

{

    ios_base::sync_with_stdio(0);

    cin.tie(0); cout.tie(0);



    memset(dp, -1, sizeof dp);



    cin >> n >> m;



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

        int a, b; cin >> a >> b;



        graph[a].pb(b);

    }



    ll ans = 0;

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

        ans = max(ans, dfs(i) + 1);

    }



    cout << ans << "\n";



    return 0;

}


// Faster C++ solution for the same problem (measured speedup: 4.311290119595068x):
#include<bits/stdc++.h>

#define ll          long long int

#define ld          long double

#define pb          push_back

#define mp          make_pair

#define ff          first

#define se          second

#define scan(n)     scanf("%lld", &n);

#define print(n)    printf("%lld\n", n);

#define fd(n)       fixed<<setprecision(n)

#define MAX         1000000000000000000

#define MAXN        100005

#define LOG         19

#define mod         1000000007

using namespace std;





int main(){

    ios_base::sync_with_stdio(false);

    cin.tie(NULL);

    ll n,m,i,x,y,mx=0;

    cin>>n>>m;

    vector<ll> vi[n+1];

    ll deg[n+1];

    queue<ll> qe;

    vector<ll> vi1;

    memset(deg,0,sizeof(deg));

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

        cin>>x>>y;

        vi[x].pb(y);

        deg[y]++;

    }

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

        if(deg[i]==0){

            qe.push(i);

            vi1.pb(i);

        }

    }

    while(!qe.empty()){

        ll node=qe.front();

        qe.pop();

        for(auto x:vi[node]){

            deg[x]--;

            if(deg[x]==0){

                vi1.pb(x);

                qe.push(x);

            }

        }

    }

    ll dp[n+1];

    memset(dp,0,sizeof(dp));

    for(auto node:vi1){

        for(auto x:vi[node]){

            dp[x]=max(dp[x],dp[node]+1);

        }

    }

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

        mx=max(mx,dp[i]);

    }

    cout<<mx;

}