#pragma GCC optimize("Ofast")
#pragma GCC optimize ("unroll-loops")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")

#include <bits/stdc++.h>
using namespace std;

#ifdef DEBUG
#define n_l '\n'
#define dbg(...) cout << "[" << #__VA_ARGS__ << "]: "; cout << to_string(__VA_ARGS__) << endl
template <typename T, size_t N> int SIZE(const T (&t)[N]){ return N; } template<typename T> int SIZE(const T &t){ return t.size(); } string to_string(const string s, int x1=0, int x2=1e9){ return '"' + ((x1 < (int) s.size()) ? s.substr(x1, x2-x1+1) : "") + '"'; } string to_string(const char* s) { return to_string((string) s); } string to_string(const bool b) { return (b ? "true" : "false"); } string to_string(const char c){ return string({c}); } template<size_t N> string to_string(const bitset<N> &b, int x1=0, int x2=1e9){ string t = ""; for(int __iii__ = min(x1,SIZE(b)),  __jjj__ = min(x2, SIZE(b)-1); __iii__ <= __jjj__; ++__iii__){ t += b[__iii__] + '0'; } return '"' + t + '"'; } template <typename A, typename... C> string to_string(const A (&v), int x1=0, int x2=1e9, C... coords); int l_v_l_v_l = 0, t_a_b_s = 0; template <typename A, typename B> string to_string(const pair<A, B> &p) { l_v_l_v_l++; string res = "(" + to_string(p.first) + ", " + to_string(p.second) + ")"; l_v_l_v_l--; return res; } template <typename A, typename... C> string to_string(const A (&v), int x1, int x2, C... coords) { int rnk = rank<A>::value; string tab(t_a_b_s, ' '); string res = ""; bool first = true; if(l_v_l_v_l == 0) res += n_l; res += tab + "["; x1 = min(x1, SIZE(v)), x2 = min(x2, SIZE(v)); auto l = begin(v); advance(l, x1); auto r = l; advance(r, (x2-x1) + (x2 < SIZE(v))); for (auto e = l; e != r; e = next(e)) { if (!first) { res += ", "; } first = false; l_v_l_v_l++; if(e != l){ if(rnk > 1) { res += n_l; t_a_b_s = l_v_l_v_l; }; } else{ t_a_b_s = 0; } res += to_string(*e, coords...); l_v_l_v_l--; } res += "]"; if(l_v_l_v_l == 0) res += n_l; return res; } void dbgm(){;} template<typename Heads, typename... Tails> void dbgm(Heads H, Tails... T){ cout << to_string(H) << " | "; dbgm(T...); }
#define dbgm(...) cout << "[" << #__VA_ARGS__ << "]: "; dbgm(__VA_ARGS__); cout << endl
#else
#define dbg(...) ;
#define dbgm(...) ;
#endif

#define endl "\n"
#define rep(i, begin, end) for (__typeof(end) i = (begin) - ((begin) > (end)); i != (end) - ((begin) > (end)); i += 1 - (((begin) > (end)) << 1))
#define all(X) begin(X),end(X)
#define sz(x) ((int) x.size())

//#define MEM_EATER
#ifndef MEM_EATER
#define rd1(x) scanf("%d", &x)
#define rd2(x, y) scanf("%d%d", &x, &y)
#define rd3(x, y, z) scanf("%d%d%d", &x, &y, &z)
#else
#define int ll
#define rd1(x) scanf("%lld", &x)
#define rd2(x, y) scanf("%lld%lld", &x, &y)
#define rd3(x, y, z) scanf("%lld%lld%lld", &x, &y, &z)
#endif

typedef long long ll;
typedef pair<int, int> ii;
typedef vector<int> vi;

#define rdl1(x) scanf("%lld", &x)
#define rdl2(x, y) scanf("%lld%lld", &x, &y)
#define rdl3(x, y, z) scanf("%lld%lld%lld", &x, &y, &z)

vector<vi> adj;
vi indegree;
vector<bool> vis;
vi dist;

void dfs(int v) {
	vis[v] = true;
	for(int i : adj[v]) {
		indegree[i]--;
		dist[i] = max(dist[i], dist[v]+1);
		if(indegree[i] == 0)
			dfs(i);
	}
}

int32_t main() {

#ifdef DEBUG
//	freopen("input.txt", "r", stdin);
//	freopen("output.txt", "w", stdout);
#endif
	int n,m;
	cin >> n >> m;
	adj.resize(n);
	indegree.resize(n);
	vis.resize(n);
	dist.resize(n);
	while(m--) {
		int x,y;
		cin >> x >> y;
		x--; y--;
		adj[x].push_back(y);
		indegree[y]++;
	}

	rep(i,0,n)
		if(not vis[i] and indegree[i] == 0)
			dfs(i);

	int ans=0;
	rep(i,0,n)
		ans = max(ans, dist[i]);
	cout << ans << endl;

	return 0;
}


Measured memory_bytes: 9472