#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.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>;
template <typename T>
using ordered_set_rev = tree<T, null_type, greater<T>, rb_tree_tag, tree_order_statistics_node_update>;
template <typename T>
using dijkstra = priority_queue<T, vector<T>, greater<T>>;

using namespace std;
typedef long long ll;
typedef unsigned long long ull;
#define fbo find_by_order //k th index
#define ook order_of_key // strictly smaller than k
#define PI acos(-1.0)
#define inf 0x3f3f3f3f
#define max_ull 18446744073709551615
#define max_ll 9223372036854775807
#define min3(a, b, c) min(a, min(b, c))
#define max3(a, b, c) max(a, max(b, c))
#define min4(a, b, c, d) min(min(a, b), min(c, d))
#define max4(a, b, c, d) max(max(a, b), max(c, d))
#define max5(a, b, c, d, e) max(max3(a, b, c), max(d, e))
#define min5(a, b, c, d, e) min(min3(a, b, c), min(d, e))
#define lead_zero(x) __builtin_clzll(x)
#define trail_zero(x) __builtin_ctzll(x)
#define total_1s(x) __builtin_popcountll(x)
#define first_1(x) __builtin_ffsll(x)
#define log2_(x) __builtin_clz(1) - __builtin_clz(x)
#define isLeap(x) ((x % 400 == 0) || (x % 100 ? x % 4 == 0 : false))
#define QUERY int test;scanf("%d", &test);for (int _T = 1; _T <= test; _T++)
#define FAST ios_base::sync_with_stdio(0), cin.tie(0)
#define all(v) v.begin(), v.end()
#define reunique(v) v.resize(std::unique(all(v)) - v.begin())
#define pii pair<int, int>
#define pll pair<ll, ll>
#define ff first
#define ss second
#define MERGE(v1, v2, v) merge(all(v1), all(v2), back_inserter(v))
#define MP make_pair
#define EB emplace_back
#define read freopen("input.txt", "r", stdin)
#define write freopen("output.txt", "w", stdout)
#define in_range(v, r, l) upper_bound(all(v), r) - lower_bound(all(v), l)
#define LCM(a,b) (a/__gcd(a,b))*b;
ll rdn(int y, int m, int d) {
    /* Rata Die day one is 0001-01-01 */
    if (m < 3)
        y--, m += 12;
    return 365 * y + y / 4 - y / 100 + y / 400 + (153 * m - 457) / 5 + d - 306;
}
/* Direction arrays */
/* int dx[] = {1,-1,0,0}, dy[] = {0,0,1,-1}; */                        // 4 Direction
/*int dx[] = {1,-1,0,0,1,1,-1,-1}, dy[] = {0,0,1,-1,1,-1,1,-1};       */ // 8 Direction
/* int dx[] = {1,-1,1,-1,2,2,-2,-2} , dy[] = {2,2,-2,-2,1,-1,1,-1}; */ // Knight Direction
/* int dx[] = {2,-2,1,1,-1,-1} , dy[] = {0,0,1,-1,1,-1}; */            // Hexagonal Direction
/* int dx[] = {2,-2,1,1,-1,-1} , dy[] = {0,0,1,-1,1,-1}; */            // Hexagonal Direction
/* int day[] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}; */
const int N=1e5+9;
vector<int>edge[N];
int dp[N];
int recur(int x,int par){
    int ans=0;
    if(dp[x]!=-1)return dp[x];
    for(int i:edge[x]){
        if(i==par)continue;
        int t=1+recur(i,x);
        ans=max(ans,t);
    }
    return dp[x]=ans;
}
int main() {
    int n,m;
    int a,b;
    scanf("%d %d",&n,&m);
    for(int i=0;i<m;i++){
        scanf("%d %d",&a,&b);
        edge[a].emplace_back(b);
    }
    for(int i=1;i<=n;i++){
        edge[0].emplace_back(i);
    }
    memset(dp,-1,sizeof dp);
    int ans=recur(0,-1)-1;
    cout<<ans<<endl;
}


Measured memory_bytes: 9336