#include<bits/stdc++.h>
using namespace std;
int ig[114514], dfn[114514], low[114514], col[114514], vl[114514], f[114514], in[114514], vu[114514];
int n, m, a1, a2, cnt, ign, ans;
bool vis[114514];
vector<int> G[114514], NG[114514];
stack<int> s;
queue<int> q;
void tarjan(int u){
	dfn[u] = low[u] = ++cnt;
	s.push(u);
	vis[u] = 1;
	for (auto &v : G[u]){
		if (!dfn[v]){
			tarjan(v);
			low[u] = min(low[u], low[v]);
		}else if (vis[v])
			low[u] = min(low[u], dfn[v]);
	}
	if (dfn[u] == low[u]){
		while (!s.empty()){
			int v = s.top();
			s.pop();
			vis[v] = 0;
			col[v] = u;
			vl[u]++;
			if (u == v)
				break;
		}
	}
}
void dfs(int u){
	for (auto v : NG[u]){
		dfs(v);
		if (u == vu[v])
			continue;
		f[u] = max(f[u], f[v] + vl[u]);
	}
}
int main(){
	ios::sync_with_stdio(false);
	cin.tie(0);cout.tie(0);
	cin >> n >> m;
	for (int i = 1; i <= m; i++){
		int u, v;
		cin >> u >> v;
		G[u].push_back(v);
	}
	for (int i = 1; i <= n; i++)
		if (!dfn[i])
			tarjan(i);
	for (int i = 1; i <= n; i++)
		for (auto v : G[i]){
			if (col[i] == col[v])
				continue;
			NG[col[i]].push_back(col[v]);
			ig[col[i]] = ig[col[v]] = 1;
		//	cout << col[v] << " <- " << col[i] << "\n";
		}
	for (int i = 1; i <= n; i++)
		if (col[i] == i && !ig[i])
			ig[i] = 1;
	for (int i = 1; i <= n; i++)
		if (ig[i])
			ign++;
	for (int i = 1; i <= n; i++)
		for (auto v : NG[i])
			in[v]++;
	for (int i = 1; i <= n; i++)
		if (ig[i] && !in[i]){
			q.push(i);
			f[i] = vl[i];
		}
	while (!q.empty()){
		int u = q.front();
		q.pop();
		for (auto v : NG[u]){
			in[v]--;
			f[v] = max(f[u] + vl[v], f[v]);
			if (f[v] == f[u] + vl[v])
				vu[v] = u;
		}
	}
	for (int i = 1; i <= n; i++)
		if (col[i] == i)
			dfs(i);
	cout << f[col[1]];
}