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import java.io.IOException;
import java.io.InputStream;
import java.util.InputMismatchException;
import java.util.Iterator;
import java.util.LinkedList;

public class kon {
  private static int highestNodeIndex;

  public static void main(String... args) {
    FastScanner scanner = new FastScanner(System.in);

    highestNodeIndex = scanner.nextInt();
    int numberOfLines = scanner.nextInt();

    Node[] nodes = new Node[highestNodeIndex + 1];
    LinkedList<Node> graph = new LinkedList<>();
    for (int index = 1; index <= highestNodeIndex; index++) {
      Node node = new Node(index);
      nodes[index] = node;
      graph.add(node);
    }
    for (int i = 0; i < numberOfLines; i++) {
      Node source = nodes[scanner.nextInt()];
      Node target = nodes[scanner.nextInt()];
      source.targets.add(target);
    }

    LinkedList<Node> cycle = findCycle(graph);
    if (cycle == null) {
      System.out.println("NIE");
      return;
    }

    LinkedList<Integer> hubs = findHubs(cycle, graph);

    if (hubs.isEmpty()) {
      printHubs(hubs);
      return;
    }

    for (Integer hub : hubs) {
      nodes[hub].targets.clear();
    }

    cycle = findCycle(graph);
    if (cycle == null) {
      printHubs(hubs);
      return;
    } else {
      printHubs(new LinkedList<Integer>());
    }
  }

  private static void printHubs(LinkedList<Integer> hubs) {
    System.out.println(hubs.size());

    Iterator<Integer> iterator = hubs.iterator();
    if (iterator.hasNext()) {
      System.out.print(iterator.next());
    }
    while (iterator.hasNext()) {
      System.out.print(" ");
      System.out.print(iterator.next());
    }
    System.out.println();
  }

  private static LinkedList<Integer> findHubs(LinkedList<Node> cycle, LinkedList<Node> graph) {
    boolean[] isInCycle = new boolean[highestNodeIndex + 1];
    boolean[] isInPath = new boolean[highestNodeIndex + 1];
    int[] isClouded = new int[highestNodeIndex + 1];
    boolean[] canBeDetoured = new boolean[highestNodeIndex + 1];
    for (Node cycleNode : cycle) {
      isInCycle[cycleNode.index] = true;
    }

    LinkedList<Node> cloudStarters = new LinkedList<>(cycle);

    int numberOfDetours = 0;
    boolean[] isDetourEnd = new boolean[highestNodeIndex + 1];
    while (!cloudStarters.isEmpty()) {
      Node cloudStarter = cloudStarters.removeFirst();
      if (isDetourEnd[cloudStarter.index]) {
        isDetourEnd[cloudStarter.index] = false;
        numberOfDetours--;
      }
      if (numberOfDetours > 0) {
        canBeDetoured[cloudStarter.index] = true;
      }

      LinkedList<Node> path = new LinkedList<Node>();
      path.add(cloudStarter);
      isInPath[cloudStarter.index] = true;

      while (!path.isEmpty()) {
        Node tail = path.getLast();

        if (!tail.targets.isEmpty()) {
          Node target = tail.targets.removeLast();
          if (isInCycle[target.index]) {
            if (!isDetourEnd[target.index]) {
              isDetourEnd[target.index] = true;
              numberOfDetours++;
            }
            isClouded[tail.index] = target.index;
          } else if (isInPath[target.index]) {
            // another cycle!!!
            return new LinkedList<Integer>();
          } else if (isClouded[target.index] != 0) {
            int cloudedBy = isClouded[target.index];
            isClouded[tail.index] = cloudedBy;
            if (!isDetourEnd[cloudedBy]) {
              isDetourEnd[cloudedBy] = true;
              numberOfDetours++;
            }
          } else {
            isInPath[target.index] = true;
            path.add(target);
          }
        } else {
          isInPath[tail.index] = false;
          path.removeLast();
        }
      }
    }

    for (Node cycleNode : cycle) {
      if (numberOfDetours > 0) {
        if (isDetourEnd[cycleNode.index]) {
          isDetourEnd[cycleNode.index] = false;
          numberOfDetours--;
        }
      }
      if (numberOfDetours > 0) {
        canBeDetoured[cycleNode.index] = true;
      } else {
        break;
      }
    }

    LinkedList<Integer> hubs = new LinkedList<>();
    for (int i = 1; i <= highestNodeIndex; i++) {
      if (isInCycle[i] && !canBeDetoured[i]) {
        hubs.add(i);
      }
    }
    return hubs;
  }

  private static LinkedList<Node> findCycle(LinkedList<Node> graph) {
    LinkedList<Node> startCandidates = new LinkedList<>(graph);

    while (!startCandidates.isEmpty()) {
      LinkedList<Node> cycle = new LinkedList<>();
      boolean[] isInCycle = new boolean[highestNodeIndex + 1];
      cycle.add(startCandidates.removeFirst());
      isInCycle[cycle.getFirst().index] = true;

      while (!cycle.isEmpty()) {
        Node endOfCycle = cycle.getLast();
        if (endOfCycle.targets.isEmpty()) {
          cycle.removeLast();
          isInCycle[endOfCycle.index] = false;
          if (!cycle.isEmpty()) {
            cycle.getLast().targets.removeFirst();
          }
        } else {
          Node nextInCycle = endOfCycle.targets.getFirst();
          if (isInCycle[nextInCycle.index]) {
            while (cycle.getFirst() != nextInCycle) {
              cycle.removeFirst();
            }
            return cycle;
          } else {
            cycle.add(nextInCycle);
            isInCycle[nextInCycle.index] = true;
          }
        }
      }
    }
    return null;
  }

  private static class Node {
    int index;
    LinkedList<Node> targets = new LinkedList<>();

    public Node(int index) {
      this.index = index;
    }

    public String toString() {
      return "" + index;
    }
  }

  private static class FastScanner {
    private final InputStream stream;
    private final byte[] buf = new byte[1024];
    private int curChar;
    private int numChars;
    private SpaceCharFilter filter;

    public FastScanner(InputStream stream) {
      this.stream = stream;
    }

    public int read() {
      if (numChars == -1) {
        throw new InputMismatchException();
      }
      if (curChar >= numChars) {
        curChar = 0;
        try {
          numChars = stream.read(buf);
        } catch (IOException e) {
          throw new InputMismatchException();
        }
        if (numChars <= 0) {
          return -1;
        }
      }
      return buf[curChar++];
    }

    public int nextInt() {
      int c = read();
      while (isSpaceChar(c)) {
        c = read();
      }
      int sgn = 1;
      if (c == '-') {
        sgn = -1;
        c = read();
      }
      int res = 0;
      do {
        if (c < '0' || c > '9') {
          throw new InputMismatchException();
        }
        res *= 10;
        res += c - '0';
        c = read();
      } while (!isSpaceChar(c));
      return res * sgn;
    }

    public boolean isSpaceChar(int c) {
      if (filter != null) {
        return filter.isSpaceChar(c);
      }
      return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
    }

    public interface SpaceCharFilter {
      public boolean isSpaceChar(int ch);
    }
  }
}