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#include <cstdio>
#include <vector>
#include <set>
#include <queue>
using namespace std;

vector<vector<int> > roadsTo;
vector<vector<int> > roadsFrom;
set<int> nodesToAnalyze;
set<int> result;

static void readAndRemoveNonCyclic(int n, int m)
{
	vector<set<int> > to;
	vector<set<int> > from;
	queue<int> toRemove;

	to.assign(n+1, set<int>());
	from.assign(n+1, set<int>());

	int a, b;
	for (int i = 0; i < m; ++i) {
		scanf("%d %d", &a, &b);
		to[a].insert(b);
		from[b].insert(a);
	}

	for (int i = 1; i <= n; ++i) {
		if (from[i].empty() || to[i].empty()) {
			toRemove.push(i);
		}
	}

	while (!toRemove.empty()) {
		int removed = toRemove.front();
		toRemove.pop();

		for (set<int>::iterator it = from[removed].begin(); it != from[removed].end(); ++it) {
			to[*it].erase(removed);
			if (to[*it].empty()) {
				toRemove.push(*it);
			}
		}
		from[removed].erase(from[removed].begin(), from[removed].end());
		
		for (set<int>::iterator it = to[removed].begin(); it != to[removed].end(); ++it) {
			from[*it].erase(removed);
			if (from[*it].empty()) {
				toRemove.push(*it);
			}
		}
		to[removed].erase(to[removed].begin(), to[removed].end());
	}

	for (int i = 1; i <= n; ++i) {
		roadsTo[i].assign(to[i].begin(), to[i].end());
		roadsFrom[i].assign(from[i].begin(), from[i].end());
	}
}

static void prepareNodesToAnalyze(int n)
{
	vector<int> statuses;
	vector<pair<int, int> > branch;
	statuses.assign(n+1, -1);

	for (int i = 1; i <= n; ++i) {
		if (statuses[i] == -1) {
			int pos;
			statuses[i] = 0;
			branch.push_back(make_pair(i, 0));
			while (!branch.empty()) {
				pos = branch.size()-1;
				bool found = false;
				for (int j = branch[pos].second; j < roadsTo[branch[pos].first].size(); ++j) {
					int next = roadsTo[branch[pos].first][j];
					if (statuses[next] >= 0) {
						found = true;
						for (int k = statuses[next]; k < branch.size(); ++k) {
							nodesToAnalyze.insert(branch[k].first);
						}
						return;
					} else if (statuses[next] == -1) {
						branch[pos].second = j+1;
						statuses[next] = pos+1;
						branch.push_back(make_pair(next, 0));
						found = true;
						break;
					}
				}
				if (!found) {
					statuses[branch[pos].first] = -5;
					branch.pop_back();
				}
			}
		}
	}
}

static bool findCycleWithoutNode(int n, int node) 
{
	vector<int> statuses;
	vector<pair<int, int> > branch;
	statuses.assign(n+1, -1);

	int lStart = 1;
	int lEnd = n;
	if (!result.empty()) {
		lStart = lEnd = *(result.begin());
	}

	statuses[node] = -5;

	for (int i = lStart; i <= lEnd; ++i) {
		if (statuses[i] == -1) {
			int pos;
			statuses[i] = 0;
			branch.push_back(make_pair(i, 0));
			while (!branch.empty()) {
				pos = branch.size()-1;
				bool found = false;
				for (int j = branch[pos].second; j < roadsTo[branch[pos].first].size(); ++j) {
					int next = roadsTo[branch[pos].first][j];
					if (statuses[next] >= 0) {
						vector<int> toRemove;

						for (set<int>::iterator it = nodesToAnalyze.begin(); it != nodesToAnalyze.end(); ++it) {
							if (statuses[*it] < 0) {
								toRemove.push_back(*it);
							}
						}
						for (vector<int>::iterator it = toRemove.begin(); it != toRemove.end(); ++it) {
							nodesToAnalyze.erase(*it);
						}

						return true;
					} else if (statuses[next] == -1) {
						branch[pos].second = j+1;
						statuses[next] = pos+1;
						branch.push_back(make_pair(next, 0));
						found = true;
						break;
					}
				}
				if (!found) {
					statuses[branch[pos].first] = -5;
					branch.pop_back();
				}
			}
		}
	}
	return false;
}

static void checkNode(int n, int node) {
	if (!findCycleWithoutNode(n, node)) {
		result.insert(node);
		nodesToAnalyze.erase(node);

		int t = node;
		while (roadsTo[t].size() == 1) {
			t = roadsTo[t][0];
			if (nodesToAnalyze.erase(t) == 1) {
				result.insert(t);
			} else {
				break;
			}
		}
		t = node;
		while (roadsFrom[t].size() == 1) {
			t = roadsFrom[t][0];
			if (nodesToAnalyze.erase(t) == 1) {
				result.insert(t);
			} else {
				break;
			}
		}
	}
}

int main()
{
	int n, m;

	scanf("%d %d", &n, &m);
	roadsTo.assign(n+1, vector<int>());
	roadsFrom.assign(n+1, vector<int>());

	readAndRemoveNonCyclic(n, m);
	prepareNodesToAnalyze(n);

	if (!nodesToAnalyze.empty()) {
		while (!nodesToAnalyze.empty()) {
			checkNode(n, *(nodesToAnalyze.begin()));
		}

		printf("%d\n", result.size());
		for (set<int>::iterator it = result.begin(); it != result.end(); ++it) {
			printf("%d ", *it);
		}
		printf("\n");
	} else {
		printf("NIE\n");
	}

	return 0;
}