using namespace std;

const int N = 10010, M = 50050;
int n, m;
int scc_cnt;
vector<int> G[N];
int dfn[N], low[N], timestamp, stk[N], top;
int dout[N], id[N], size[N];
bool in_stk[N];

void add(int a, int b){
	G[a].push_back(b);
}

void tarjan(int u){
	dfn[u] = low[u] = timestamp++;
	stk[++top] = u, in_stk[u] = true;
	for(int i : G[u]){
		if(!dfn[i]){
			tarjan(i);
			low[u] = min(low[u],low[i]);
		}
		else if(in_stk[i]) low[u] = min(low[u],dfn[i]);
	}
	if(dfn[u]==low[u]){
		++scc_cnt;
		int y;
		do{
			y = stk[top--];
			in_stk[y] = false;
			id[y] = scc_cnt;
			size[scc_cnt]++;
		}while(y!=u);
	}
}
int main(){
	cin>>n>>m;
	
	int a, b;
	for(int i=0;i<m;i++){
		scanf("%d%d",&a,&b);
		add(a,b);
	}
	
	for(int i=1;i<=n;i++){
		if(!dfn[i]){
			tarjan(i);
		}
	}
	
	for(int i=1;i<=n;i++){
		for(int j : G[i]){
			int a = id[i], b = id[j];
			if(a!=b) dout[a]++;
		}
	}
	
	int zeros = 0, sum = 0;
	for(int i=1;i<=scc_cnt;i++){
		if(!dout[i]){
			zeros++;
			sum+=size[i];
			if(zeros>1){
				sum = 0;
				break;
			}
		}
	}
	printf("%d\n",sum);
	
	return 0;
}