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import java.awt.Color;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.StreamTokenizer;
import java.util.Comparator;
import java.util.HashSet;
import java.util.Set;

public class kon {

	static int time = 0;

	static Set<City> result = null;

	static boolean sayNo = false;

	static {
		time = 0;
		sayNo = false;
		result = null;
	}

	private static class City {
		final int id;

		int timeIn;

		int timeOut;

		Set<City> neighbours;

		Color color;

		City parent;

		public City(int id) {
			this.id = id;
			neighbours = new HashSet<>();
			color = Color.WHITE;
			parent = null;
		}

		@Override
		public int hashCode() {
			return id;
		}

		@Override
		public boolean equals(Object obj) {
			if (this == obj)
				return true;
			if (obj == null)
				return false;
			if (getClass() != obj.getClass())
				return false;
			City other = (City) obj;
			if (id != other.id)
				return false;
			return true;
		}

	}

	public static void main(String[] args) {
		// TODO Auto-generated method stub

		makeResult();
		System.exit(0);

	}

	private static void makeResult() {
		BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(System.in));
		StreamTokenizer st = new StreamTokenizer(bufferedReader);
		try {
			st.nextToken();
			int n = (int) st.nval;
			st.nextToken();
			int m = (int) st.nval;

			City[] cities = new City[n + 1];
			for (int i = 1; i <= n; i++) {
				cities[i] = new City(i);
			}
			int a, b = 0;
			for (int i = 1; i <= m; i++) {
				st.nextToken();
				a = (int) st.nval;
				st.nextToken();
				b = (int) st.nval;
				cities[a].neighbours.add(cities[b]);
			}
			bufferedReader.close();

			for (int i = 1; i <= n; i++) {
				if (cities[i].color == Color.WHITE) {
					dfsVisit(cities[i]);
					if (sayNo) {
						System.out.println(0);
						return;
					}
				}
			}

			if (result == null) {
				System.out.println("NIE");
				return;
			} else {
				System.out.println(result.size());
				result.stream().sorted(Comparator.comparing(x -> x.id)).forEach(x -> System.out.print(x.id + " "));
			}

		} catch (Exception e) {
			e.printStackTrace();
		}
	}

	private static void dfsVisit(City c) {
		c.color = Color.GRAY;
		c.timeIn = time + 1;
		time = time + 1;

		City ancestor = null;
		for (City city : c.neighbours) { // main procedure
			if (city.color == Color.GRAY && ((ancestor == null) || (ancestor.timeIn < city.timeIn))) {
				ancestor = city;
			}
		}
		if (ancestor != null) { // there is a cycle
			Set<City> cycle = new HashSet<>();
			cycle.add(ancestor);
			City tmp = c;
			while (tmp != ancestor) {
				cycle.add(tmp);
				tmp = tmp.parent;
			}
			if (result == null) {
				result = cycle;
			} else {
				result.retainAll(cycle);
				if (result.isEmpty()) {
					sayNo = true;
					return;
				}
			}
		}

		for (City city : c.neighbours) { // main procedure
			if (city.color == Color.WHITE) {
				city.parent = c;
				dfsVisit(city);
				if (sayNo) {
					return;
				}
			}
		}
		// here, case when I foun black one!?!?!?

		c.color = Color.BLACK;
		c.timeOut = time + 1;
	}

}