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#include <algorithm>
#include <cstdio>
#include <stack>
#include <vector>
using namespace std;

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define VAR(v,w) __typeof(w) v=(w)
#define FORE(it,c) for(VAR(it,(c).begin());it!=(c).end();++it)
#define PB push_back
#define ALL(c) (c).begin(),(c).end()

typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<bool> VB;

int n, m;
VVI g;
bool belong[500000], ent[500000], ex[500000], rr[500000];
VI l;
int aFwd[500000], bFwd[500000], aBack[500000], bBack[500000];

class SCC {
	int ind;
	VI index, low;
	stack<int> s;
	VB ins;
	void dfs(int v) {
		if (res[v] >= 0)
			return;
		index[v] = low[v] = ind++;
		s.push(v);
		ins[v] = 1;
		FORE(it,g[v]) {
			int w = *it;
			if (index[w] < 0) {
				dfs(w);
				low[v] = min(low[v], low[w]);
			} else if (ins[w])
				low[v] = min(low[v], index[w]);
		}
		if (low[v] == index[v]) {
			int w;
			do {
				w = s.top();
				s.pop();
				ins[w] = 0;
				res[w] = k;
			} while (w != v);
			++k;
		}
	}
public:
	VI res;
	int k;
	SCC() : ind(0), index(n, -1), low(n, -1), s(), ins(n), res(n, -1), k(0) {
		REP(i,n)
			dfs(i);
	}
};

void go(int v) {
	ent[v] = 1;
	FORE(it,g[v]) {
		int w = *it;
		if (!belong[w]) continue;
		if (ent[w] && !ex[w]) {
			++aBack[v];
			++bBack[w];
			continue;
		}
		++aFwd[v];
		++bFwd[w];
		if (!ent[w]) go(w);
	}
	ex[v] = 1;
	l.PB(v);
}

int main() {
	scanf("%d%d", &n, &m);
	g.resize(n);
	REP(j,m) {
		int a, b;
		scanf("%d%d", &a, &b);
		--a;
		--b;
		g[a].PB(b);
	}
	SCC scc;
	VI c(scc.k);
	FORE(it,scc.res) ++c[*it];
	int comp = -1;
	REP(k,scc.k) if (c[k] > 1) {
		if (comp >= 0) {
			printf("0\n\n");
			return 0;
		}
		comp = k;
	}
	if (comp < 0) {
		printf("NIE\n");
		return 0;
	}
	REP(i,n) belong[i] = scc.res[i] == comp;
	REP(i,n) if (belong[i] && !ex[i]) go(i);
	reverse(ALL(l));
	int fwd = 0, back = 0, backTotal = 0;
	FORE(it,l) backTotal += aBack[*it];
	int r = 0;
	FORE(it,l) {
		fwd -= bFwd[*it];
		back += bBack[*it];
		if (!fwd && back == backTotal) {
			rr[*it] = 1;
			++r;
		}
		back -= aBack[*it];
		fwd += aFwd[*it];
	}
	printf("%d\n", r);
	bool first = 1;
	REP(i,n) if (rr[i]) {
		printf(first ? "%d" : " %d", i + 1);
		first = 0;
	}
	printf("\n");
}