#include<bits/stdc++.h>
using namespace std;
const int N = 20100;
struct Tarjan_scc {
	stack<int> s;
	bool instack[N];
	struct edge { int to, next; }G[N * 3];
	int idx, scc, tot, in[N], out[N], dfn[N], low[N], head[N], sccnum[N];
	inline void init(int n) {
		idx = scc = tot = 0;
		while (!s.empty()) s.pop();
		for (int i = 0; i < n + 2; i++) {
			head[i] = -1;
			instack[i] = false;
			in[i] = out[i] = dfn[i] = low[i] = sccnum[i] = 0;
		}
	}
	inline void add_edge(int u, int v) {
		G[tot].to = v, G[tot].next = head[u], head[u] = tot++;
	}
	inline void built(int m) {
		int u, v;
		while (m--) {
			scanf("%d %d", &u, &v);
			add_edge(u, v);
		}
	}
	inline void tarjan(int u) {
		dfn[u] = low[u] = ++idx;
		instack[u] = true;
		s.push(u);
		for (int i = head[u]; ~i; i = G[i].next) {
			int &v = G[i].to;
			if (!dfn[v]) {
				tarjan(v);
				low[u] = min(low[u], low[v]);
			} else if (instack[v] && dfn[v] < low[u]) {
				low[u] = dfn[v];
			}
		}
		if (dfn[u] == low[u]) {
			int v = 0;
			scc++;
			do {
				v = s.top(); s.pop();
				instack[v] = false;
				sccnum[v] = scc;
			} while (u != v);
		}
	}
	inline void solve(int n, int m) {
		init(n);
		built(m);
		for (int i = 1; i <= n; i++) {
			if (!dfn[i]) tarjan(i);
		}
		int x1 = 0, x2 = 0;
		for (int u = 1; u <= n; u++) {
			for (int i = head[u]; ~i; i = G[i].next) {
				int v = G[i].to;
				if (sccnum[u] != sccnum[v]) {
					in[sccnum[v]]++;
					out[sccnum[u]]++;
				}
			}
		}
		for (int i = 1; i <= scc; i++) {
			if (!in[i]) x1++;
			if (!out[i]) x2++;
		}
		printf("%d\n", 1 == scc ? 0 : max(x1, x2));
	}
}go;
int main() {
#ifdef LOCAL
	freopen("in.txt", "r", stdin);
	freopen("out.txt", "w+", stdout);
#endif
	int n, m;
	while (~scanf("%d %d", &n, &m)) {
		go.solve(n, m);
	}
	return 0;
}