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#include <iostream>
#include <vector>
#include <queue>
#include <algorithm>
#include <set>

using namespace std;

struct Node;
typedef vector<Node *> Nodes;

struct Node
{
	Node () : scc(-1), visited(false), in(0), iter(-1), started(false) {}
	Nodes neighbours;
	Nodes parents;
	int scc;
	int id;
	bool visited;
	unsigned int in;
	int iter;
	bool started;
};

typedef vector<Node> Graph;


void getTimes( Node *n, Nodes &times )
{
	if (n->visited) return;
	n->visited = true;
	for (auto next: n->neighbours)
	{
		if (!next->visited) getTimes(next, times);
	}
	times.push_back(n);
}

void getSCC( Node *n, vector<int> &sccs, int i )
{
	if (n->scc != -1) return;
	n->scc = i;
	sccs[i]++;
	for (auto next: n->parents)
	{
    if (next->scc == -1) getSCC(next, sccs, i);
	}
}

int SCC( Graph &g )
{
	int gsize = g.size();
	Nodes times;
	times.reserve (g.size());
	for (int i = 0; i < gsize; i++)
	{
		if (!g[i].visited)
		{
			getTimes(&g[i], times);
		}
	}

	vector<int> sccs(gsize, 0);
	for (int i = gsize-1; i >= 0; i--)
	{
		if (times[i]->scc == -1)
		{
			getSCC(times[i], sccs, i);
		}
	}

	int gotOne = -2;
	for (int i = 0; i < gsize; i++)
	{
    if (sccs[i] > 1)
		{
			if (gotOne > 0) return -1;
			gotOne = i;
		}
	}

	return gotOne;
}

void clearEdges(Nodes &nodes, int scc)
{
	nodes.erase(remove_if(nodes.begin(), nodes.end(),
			 [&scc] (const Node *n) { return n->scc != scc; }), nodes.end());
}

void getResult(Graph &g, Nodes &resultNodes, int scc)
{
	Node *startNode = 0;
	for (auto &n: g)
	{
		if (n.scc == scc)
		{
			clearEdges(n.neighbours, scc);
			clearEdges(n.parents, scc);
		}
	}
	// come on, there must be a startNode!
	// now, graph starting with startNode contains only
  // single strongly connected component

	// first, find a node that have neighbours with 1 out and 1 in degree
	for (auto &n: g)
	{
		if (n.scc == scc)
		{
			bool oneOut = false;
			for (auto &next : n.neighbours)
			{
				if (next->parents.size() == 1) oneOut = true;
			}
			for (auto &next : n.parents)
			{
				if (next->neighbours.size() == 1 && oneOut)
				{
					startNode = &n;
					break;
				}
			}
			if (startNode != 0) break;
		}
	}
	if (startNode == 0) 
	{
		// surely there is no common crossroad
		return;
	}
	set<Node *> reached;
	// try topological sort;
	queue<Node *> q;
	bool found = false;
	while (!found)
	{
		while (!q.empty())
			q.pop();
		q.push(startNode);
		startNode->started = true;
		while (!q.empty() && !found)
		{
			Node *n = q.front();
			q.pop();
			reached.erase(n);
			for (auto &next : n->neighbours)
			{
				if (next->iter != startNode->id)
				{
					next->in = 0;
					next->iter = startNode->id;
				}
				next->in++;
				if (next->in == next->parents.size())
				{
					if (next==startNode)
					{
						if (q.empty())
						{
							found = true;
							// we got one!
							resultNodes.push_back(next);
						}
					}
					else
					{
						q.push(next);
					}
				}
				else
				{
					if (!next->started)
					{
						reached.insert(next);
					}
				}
			}
		}
	// if failed - get one of reached nodes, and try again, until it succeeds
		if (!found)
		{
			if (!reached.empty())
  		  startNode = *(reached.begin());
			else
				return;
		}
		// could it be that we would not find vertex?
	}
	// there is first result; already in resultNodes
	// then, flush for next results
	while (!q.empty())
		q.pop();
	q.push(resultNodes[0]);
	int gsize = g.size();
	unsigned int openEdges = 0;
	resultNodes[0]->in = openEdges;
	while (!q.empty())
	{
		Node *n = q.front();
		q.pop();
		openEdges -= n->in;
		for (auto &next : n->neighbours)
		{
			if (next->iter != gsize)
			{
				next->in = 0;
				next->iter = gsize;
			}
			next->in++;
			openEdges++;
		}
		for (auto &next : n->neighbours)
		{
			if (next->in == next->parents.size())
			{
				if (next != resultNodes[0])
				{
					q.push(next);
					if (openEdges == next->in)
						resultNodes.push_back(next);
				}
			}
		}
	}
}

int main()
{
	ios_base::sync_with_stdio(false);

	int N, M;
	cin >> N >> M;

	Graph g(N);
	for (int i = 0; i < N; i++)
		g[i].id = i;

	for (int i = 0; i < M; i++)
  {
		int a, b;
		cin >> a >> b;
		g[a-1].neighbours.push_back(&g[b-1]);
		g[b-1].parents.push_back(&g[a-1]);
	}

	int res = SCC(g);
	if (-2 == res) cout << "NIE" << endl;
	else
	{
		Nodes resultNodes;
		if (res >=0) getResult(g, resultNodes, res);
		else res = 0;
		cout << resultNodes.size() << endl;
		sort(resultNodes.begin(), resultNodes.end(),
			 	[] (const Node *a, const Node *b) { return a->id < b->id; });
		for_each(resultNodes.begin(), resultNodes.end(),
				[] (const Node *n) { cout << n->id+1 << " "; });
		cout << endl;
	}

	return 0;
}