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import java.util.*;

public class kon {
    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        int nodes = s.nextInt();
        int edges = s.nextInt();
        List<Integer>[] graph = new List[nodes];
        List<Integer> empty = new ArrayList<>();
        Arrays.fill(graph, empty);

        for (int i = 0; i < edges; i++) {
            int from = s.nextInt() - 1, to = s.nextInt() - 1;
            List<Integer> destinations = graph[from];
            if (destinations == empty) {
                destinations = new ArrayList<>();
                graph[from] = destinations;
            }
            destinations.add(to);
        }

        List<List<Integer>> scc = new Tarjan().getSCComponents(graph);
        if (scc.isEmpty()) {
            System.out.println("NIE");
        } else if (scc.size() > 1) {
            System.out.println("0");
        } else {
            List<Integer> c = scc.get(0);
            Set<Integer> anotherName = new HashSet<>(c);
            toCheck = new HashSet<>(c);
            candidates = new HashSet<>(c);
            while (!toCheck.isEmpty()) {
                blocked = new boolean[graph.length];
                are = new List[graph.length];
                stack = new ArrayList<>();
                for (int i = 0; i < nodes; i++) {
                    are[i] = new ArrayList<>();
                }
                int i = toCheck.iterator().next();
                findCycles(i, i, graph, anotherName);
            }

            if (candidates.isEmpty()) {
                System.out.println("0");
            } else {
                System.out.println(candidates.size());
                List<Integer> sorted = new ArrayList<>(candidates);
                Collections.sort(sorted);
                StringJoiner sj = new StringJoiner(" ");
                for (int i : sorted) {
                    sj.add(Integer.toString(i + 1));
                }
                System.out.println(sj);
            }
        }
    }

    static Set<Integer> candidates;
    static Set<Integer> toCheck;
    static List<Integer> stack;
    static boolean[] blocked;
    static List[] are;

    private static boolean findCycles(int v, int s, List<Integer>[] graph, Set<Integer> scc) {
        boolean f = false;
        stack.add(v);
        blocked[v] = true;

        for (int w : graph[v]) {
            if (!scc.contains(w)) continue;

            if (w == s) {
                List<Integer> cycle = new ArrayList<>();
                for (int j = 0; j < stack.size(); j++) {
                    int index = stack.get(j);
                    cycle.add(index);
                    toCheck.remove(index);
                }
                candidates.retainAll(cycle);
                f = true;
            } else if (!blocked[w]) {
                if (findCycles(w, s, graph, scc)) {
                    f = true;
                }
            }
        }

        if (f) {
            unblock(v);
        } else {
            for (int i = 0; i < graph[v].size(); i++) {
                int w = graph[v].get(i);
                if (!are[w].contains(v)) {
                    are[w].add(v);
                }
            }
        }

        stack.remove((Object) v);
        return f;
    }

    static private void unblock(int node) {
        blocked[node] = false;
        List Bnode = are[node];
        while (Bnode.size() > 0) {
            Integer w = (Integer) Bnode.get(0);
            Bnode.remove(0);
            if (blocked[w]) {
                unblock(w);
            }
        }
    }


    static class Tarjan {
        private int V;
        private int preCount;
        private int[] low;
        private boolean[] visited;
        private List<Integer>[] graph;
        private List<List<Integer>> sccComp;
        private Stack<Integer> stackTar;

        public List<List<Integer>> getSCComponents(List<Integer>[] graph) {
            V = graph.length;
            this.graph = graph;
            low = new int[V];
            visited = new boolean[V];
            stackTar = new Stack<Integer>();
            sccComp = new ArrayList<>();

            for (int v = 0; v < V; v++)
                if (!visited[v])
                    dfs(v);

            return sccComp;
        }

        public void dfs(int v) {
            low[v] = preCount++;
            visited[v] = true;
            stackTar.push(v);
            int min = low[v];
            for (int w : graph[v]) {
                if (!visited[w])
                    dfs(w);
                if (low[w] < min)
                    min = low[w];
            }
            if (min < low[v]) {
                low[v] = min;
                return;
            }
            List<Integer> component = new ArrayList<>();
            int w;
            do {
                w = stackTar.pop();
                component.add(w);
                low[w] = V;
            } while (w != v);
            if (component.size() > 1)
                sccComp.add(component);
        }

    }
}