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#include <iostream>
#include <vector>
#include <algorithm>
#include <unordered_set>

class graph
{
public:
	graph(int _count)
		: g(_count)
	{ }

	void add_edge(int _v1, int _v2)
	{
		g[_v1].n.push_back(_v2);
	}

	void topo_dfs(int _v)
	{
		if (!g[_v].t) {
			g[_v].t = 1;
			for (auto &&e : g[_v].n) {
				if (e != excluded) {
					topo_dfs(e);
				}
			}
			g[_v].t = --topo;
		}
	}

	void topo_sort()
	{
		for (auto &&v : g) {
			v.t = 0;
		}
		topo = g.size();
		for (int i = topo - 1; i >= 0; --i) {
			if (i != excluded) {
				topo_dfs(i);
			}
		}
	}

	bool acyclic_exclude(int _excluded)
	{
		excluded = _excluded;
		topo_sort();

		for (int i = 0; i < g.size(); ++i) {
			if (i == excluded) continue;
			for (auto &&e : g[i].n) {
				if (e == excluded) continue;
				if (g[i].t >= g[e].t) {
					return false;
				}
			}
		}
		return true;
	}


	void cycle_dfs(int _v)
	{
		if (g[_v].t2 == 0) {
			g[_v].t2 = 1;
			
			for (auto &&v : g[_v].n) {
				if (cf >= 0 || cf == -2) break;
				if (g[v].t2 == 1) {
					cf = v;
					break;
				}
				cycle_dfs(v);
			}

			g[_v].t2 = 2;

			if (cf >= 0) {
				c.push_back(_v);
				if (cf == _v) {
					cf = -2;
				}
			}
		}
	}

	void cycle()
	{
		t = 0;
		cf = -1;
		for (int i = 0; i < g.size(); ++i) {
			if (cf != -2) {
				cycle_dfs(i);
			} else {
				break;
			}
		}
		std::reverse(c.begin(), c.end());
	}

	// -----

	struct V
	{
		int t = -1;
		int t2 = 0;
		std::vector<int> n;
	};

	std::vector<V> g;
	std::vector<int> c;
	int t;
	int cf;
	int topo;
	int excluded;
};




int main()
{
	int n, m;
	std::cin >> n >> m;
	graph g(n);
	for (int i = 0; i < m; ++i) {
		int a, b;
		std::cin >> a >> b;
		g.add_edge(a - 1, b - 1);
	}

	if (g.acyclic_exclude(-1)) {
		std::cout << "NIE" << std::endl;
	} else {
		g.cycle();
		std::vector<int> result;
		for (auto &&c : g.c) {
			if (g.acyclic_exclude(c)) {
				result.push_back(c + 1);
			}
		}
		std::cout << result.size() << std::endl;
		std::sort(result.begin(), result.end());
		for (auto &&v : result) {
			std::cout << v << " ";
		}
	}
	return 0;
}