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#include <cstdio>
#include <vector>
#include <deque>
#include <unordered_set>
#include <algorithm>

using namespace std;

int N, M, a, b, minId, index;
bool fin, cyc;
deque<int> s;
unordered_set<int> cv, cr, ct;
vector<int> res, scc;

struct vs {
	unordered_set<int> e, b;
	int index, lowlink;
	bool onStack, blocked;
};
vector<vs> v;

void strongconnect(int vi) {
	int wi;

	v[vi].index = v[vi].lowlink = index++;
	s.push_back(vi);
	v[vi].onStack = true;

	for (auto itE = v[vi].e.begin(); !fin && itE != v[vi].e.end();) {
		int wi = *itE;
		if (wi < minId) {
			itE = v[vi].e.erase(itE);
		} else {
			if (!v[wi].index) {
				strongconnect(wi);
				if (v[wi].lowlink < v[vi].lowlink) {
					v[vi].lowlink = v[wi].lowlink;
				}
			} else if (v[wi].onStack) {
				if (v[wi].index < v[vi].lowlink) {
					v[vi].lowlink = v[wi].index;
				}
			}
			itE++;
		}
	}

	if (v[vi].lowlink == v[vi].index) {
		// strongly connected
		scc.clear();
		do {
			wi = s.back(); s.pop_back();
			v[wi].onStack = false;
			scc.push_back(wi);
		} while (wi != vi);

		if (scc.size() > 1) {
			if (cv.size() > 0) {
				fin = true;
			} else {
				cv.insert(scc.begin(), scc.end());
			}
		}
	}
}

void unblock(int vi) {
	int wi;
	v[vi].blocked = false;

	for (auto itB = v[vi].b.begin(); itB != v[vi].b.end(); itB++) {
		wi = *itB;
		if (v[wi].blocked) {
			unblock(wi);
		}
	}
	v[vi].b.clear();
}

bool circuit(int vi) {
	bool f = false;
	int wi, si;

	s.push_back(vi);
	v[vi].blocked = true;

	for (auto itE = v[vi].e.begin(); itE != v[vi].e.end(); itE++) {
		wi = *itE;
		if (wi == minId) {
			cyc = true;
			ct.clear();
			for (auto itS = s.begin(); itS != s.end(); itS++) {
				si = *itS;
				if (cr.find(si) != cr.end()) {
					ct.insert(si);
				}
			}
			cr = ct;

			f = true;
		} else if (!v[wi].blocked) {
			if (circuit(wi)) {
				f = true;
			}
		}
	}

	if (f) {
		unblock(vi);
	} else {
		for (auto itE = v[vi].e.begin(); itE != v[vi].e.end(); itE++) {
			wi = *itE;
			if (v[wi].b.find(vi) == v[wi].b.end()) {
				v[wi].b.insert(vi);
			}
		}
	}

	s.pop_back();
	return f;
}

int main() {
	scanf("%d %d", &N, &M);
	v.resize(N + 1);
	for (int i = 0; i < M; i++) {
		scanf("%d %d", &a, &b);
		v[a].e.insert(b);
	}


	// Johnson
	minId = 1;
	s.clear();
	for (int i = 1; i <= N; i++) {
		cr.insert(i);
	}

	while (!fin && minId < N) {
		// clean
		for (int i = minId; i <= N; i++) {
			v[i].index = v[i].lowlink = 0;
			v[i].onStack = false;
		}

		// Tarjan
		index = 1;
		cv.clear();
		for (int i = minId; !fin && i <= N; i++) {
			if (!v[i].index) {
				strongconnect(i);
			}
		}

		if (!fin && cv.size() > 0) {
			minId = N;
			for (auto itV = cv.begin(); itV != cv.end(); itV++) {
				int vi = *itV;
				if (vi < minId) {
					minId = vi;
				}
				v[vi].b.clear();
				v[vi].blocked = false;
			}

			circuit(minId);
			minId++;
		} else {
			minId = N;
		}
	}


	if (fin) {
		printf("0\n\n");
	} else if (!cyc) {
		printf("NIE\n");
	} else {
		res.insert(res.end(), cr.begin(), cr.end());
		sort(res.begin(), res.end());
		printf("%d\n", (int)res.size());
		for (auto itR = res.begin(); itR != res.end(); itR++) {
			printf("%d ", *itR);
		}
		printf("\n");
	}

	return 0;
}