#include <bits/stdc++.h>
#include <atcoder/all>
#define rep(i, l, n) for (int i = (l); i < (n); i++)
#define max(p, q) ((p) > (q) ? (p) : (q))
using namespace std;
using namespace atcoder;
using ll = long long;
template <class T>
using V = vector<T>;
template <class T>
using VV = V<V<T> >;


int bool_sum(V<bool>&a) {
	int res = 0;
	rep(i, 0, a.size()) {
		if (a[i]) {
			res++;
		}
	}
	return res;
}


VV<int> SCC_to_DAG(int n, VV<int>&route, VV<int>&scc) {
	VV<int> dag(scc.size(), V<int>(0));
	V<int> group(n);
	rep(i, 0, scc.size()) {
		rep(j, 0, scc[i].size()) {
			group[scc[i][j]] = i;
		}
	}

	rep(i, 0, n) {
		rep(j, 0, route[i].size()) {
			int a = group[i];	int b = group[route[i][j]];
			if (a != b) {
				dag[a].push_back(b);
			}
		}
	}

	return dag;
}


int main(void) {
	int n, m;	cin >> n >> m;

	VV<int> route(n, V<int>(0));
	scc_graph ss(n);
	rep(i, 0, m) {
		int a, b;   cin >> a >> b;
		a--;	b--;
		ss.add_edge(a, b);
		route[a].push_back(b);
	}

	VV<int> scc = ss.scc();
	if (scc.size() <= 1) {
		cout << 0 << endl;
		return 0;
	}

	VV<int> dag = SCC_to_DAG(n, route, scc);
	int ds = dag.size();
	V<bool> source(ds, true);	V<bool> sink(ds, true);
	rep(i, 0, ds) {
		rep(j, 0, dag[i].size()) {
			source[i] = sink[dag[i][j]] = false;
		}
	}
	int ans = max(bool_sum(source), bool_sum(sink));

	cout << ans << endl;
	return 0;
}