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#include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
using namespace std;

const int N = 500007;
int n, m;
vector <int> G[N];
vector <int> res;
vector <int> incip;
int stg[N];
int st[N];

bool check(int v) {
	for(int i = 1; i <= n; ++i) st[i] = stg[i];
	st[v] = 0;
	vector <int> q;
	if(stg[v] != 0) q.push_back(v);
	for(int i = 0; i < incip.size(); ++i) q.push_back(incip[i]);
	for(int i = 0; i < q.size(); ++i) {
		int u = q[i];
		for(int j = 0; j < G[u].size(); ++j) {
			st[G[u][j]]--;
			if(st[G[u][j]] == 0) q.push_back(G[u][j]);
		}
	}

	int siz = q.size();
	if(v == 0) siz--;
	return siz == n;
}

int main() {
	ios_base::sync_with_stdio(0);
	stg[0] = 315;
	cin >> n >> m;
	for(int i = 0; i < m; ++i) {
		int a, b;
		cin >> a >> b;
		G[a].push_back(b);
		stg[b]++;
	}

	for(int i = 1; i <= n; ++i) if(stg[i] == 0) incip.push_back(i);

	if(check(0)) {
		cout << "NIE\n";
		return 0;
	}
	for(int i = 1; i <= n; ++i) if(check(i)) res.push_back(i);
	sort(res.begin(), res.end());
	cout << res.size() << endl;
	for(int i = 0; i < res.size(); ++i) cout << res[i] << ' ';
	//system("pause");
	return 0;
}