#define IA_PROB "ctc"

#include <cassert>
#include <cstdio>
#include <string>
#include <vector>
#include <list>
#include <set>
#include <map>
#include <queue>
#include <stack>

#include <algorithm>

using namespace std;

typedef int node_t;
typedef vector<list<node_t> > graph_t;

void dfs(graph_t &G, vector<bool> &seen, vector<node_t> &stack, node_t node)
{
	assert(!seen[node]);
	seen[node] = true;
	for (list<node_t>::iterator i = G[node].begin(); i != G[node].end(); i++) {
		if (!seen[*i]) {
			dfs(G, seen, stack, *i);
		}
	}
	stack.push_back(node);
}

int main()
{
	freopen(IA_PROB".in", "r", stdin);
	freopen(IA_PROB".out", "w", stdout);

	int n, m;
	scanf("%d %d", &n, &m);

	vector<bool> seen(n);
	graph_t G(n);
	vector<node_t> topo_stack;

	for (int i = 0; i < m; i++) {
		int x, y;
		scanf("%d %d", &x, &y);
		G[x-1].push_back(y-1);
	}

	for (int i = 0; i < n; i++) {
		if (!seen[i]) {
			dfs(G, seen, topo_stack, i);
		}
	}

	/* reset seen */
	fill(seen.begin(), seen.end(), false);
	/* reverse graph*/
	graph_t Gr(n);
	for (int i = 0; i < n; i++) {
		while (!G[i].empty()) {
			Gr[G[i].front()].push_back(i);
			G[i].pop_front();
		}
	}
	/* release old G */

	int nr = 0;
//	vector<stack<node_t> > sccs;
	while(!topo_stack.empty()) {
		node_t node = topo_stack.back();
		topo_stack.pop_back();
		if (!seen[node]) {
//			sccs.push_back(stack<node_t>());
//			dfs(Gr, seen, sccs.back(), node);
//			sccs.pop_back();
//			sccs.push_back(stack<node_t>());
			vector<node_t> s;
			dfs(Gr, seen, s, node);
			nr++;
		}
	}

	printf("%d\n", nr);
//	for (vector<stack<node_t> >::iterator i = sccs.begin(); i != sccs.end(); i++) {
//		while (!i->empty()) {
//			printf("%d ", i->top() + 1);
//			i->pop();
//		}
//		printf("\n");
//	}

	return 0;
}