#include<bits/stdc++.h>
using namespace std;
const int N = 100010, M = 6;
int n, m, a[N], b[N], dfn[N], low[N], cnt;
int st[N], tp, fa[N], ta[N], ct[N], dis[N][M][M], cntt[N];
int in[N], f[N][M], ans;
bool ins[N], bl[M];
vector<int>v[N];
struct node {
	int t, b;
};
vector<node>tar[N][M];
queue<int>q;
void tarjan(int x) {
	dfn[x] = low[x] = ++cnt;
	st[++tp] = x;
	ins[x] = 1;
	for (int t : v[x]) {
		if (!dfn[t]) {
			tarjan(t);
			low[x] = min(low[x], low[t]);
		} else if (ins[t])
			low[x] = min(low[x], dfn[t]);
	}
	if (low[x] == dfn[x]) {
		int y;
		while (1) {
			y = st[tp--];
			fa[y] = x;
			ins[y] = 0;
			ta[y] = ++ct[x];
			if (y == x) break;
		}
	}
}
void dfs(int st, int now, int d, int bel) {
	for (int t : v[now])
		if (fa[t] == bel && !bl[ta[t]]) {
			dis[bel][st][ta[t]] = max(dis[bel][st][ta[t]], d + 1);
			cntt[ta[t]]++;
			if (cntt[ta[t]] <= 16) {
				bl[ta[t]] = 1;
				dfs(st, t, d + 1, bel);
				bl[ta[t]] = 0;
			}
		}
}
int main() {
	cin >> n >> m;
	for (int i = 1; i <= m; i++) {
		cin >> a[i] >> b[i];
		v[a[i]].push_back(b[i]);
	}
	for (int i = 1; i <= n; i++)
		if (!dfn[i]) tarjan(i);
	for (int i = 1; i <= n; i++) {
		dis[fa[i]][ta[i]][ta[i]] = 0;
		memset(cntt, 0, sizeof(cntt));
		bl[ta[i]] = 1;
		dfs(ta[i], i, 0, fa[i]);
		bl[ta[i]] = 0;
	}
	for (int i = 1; i <= m; i++) {
		int p = fa[a[i]], q = fa[b[i]];
		if (p != q) {
			tar[p][ta[a[i]]].push_back({q, ta[b[i]]});
			in[q]++;
		}
	}
	for (int i = 1; i <= n; i++)
		if (ct[i] && !in[i]) {
			for (int j = 1; j <= ct[i]; j++) {
				for (int k = 1; k <= ct[i]; k++)
					f[i][j] = max(f[i][j], dis[i][k][j] + 1);
			}
			q.push(i);
		}
	while (!q.empty()) {
		int d = q.front();
		q.pop();
		for (int j = 1; j <= ct[d]; j++) {
			int s = f[d][j] + 1;
			for (node k : tar[d][j]) {
				for (int p = 1; p <= ct[k.t]; p++)
					f[k.t][p] = max(f[k.t][p], dis[k.t][k.b][p] + s);
				in[k.t]--;
				if (!in[k.t]) q.push(k.t);
			}
		}
	}
	for (int i = 1; i <= n; i++)
		for (int j = 1; j <= ct[i]; j++)
			ans = max(ans, f[i][j]);
	cout << ans;
	return 0;
}
/*
7 9
1 2
2 3
3 4
4 5
5 6
6 2
4 7
6 4
3 5

ans:7
*/