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// Krzysztof Małysa
#include <bits/stdc++.h>
#include <unistd.h>

using namespace std;

#define FOR(i,a,n) for (int i = (a), __n ## i = n; i < __n ## i; ++i)
#define REP(i,n) FOR(i,0,n)
#define FORD(i,a,n) for (int i = (a), __n ## i = n; i >= __n ## i; --i)
#define LET(x,a) __typeof(a) x = (a)
#define FOREACH(i,x) for (LET(i, x.begin()), __n##i = x.end(); i != __n##i; ++i)
#define ALL(x) (x).begin(), (x).end()
#define SZ(x) (int(x.size()))
#define eprintf(...) fprintf(stderr, __VA_ARGS__)
#define ST first
#define ND second
#define MP make_pair
#define PB push_back
#define O(...) ostream& operator <<(ostream& os, const __VA_ARGS__& x)
#define OO(...) O(__VA_ARGS__) { return __out(os, ALL(x)); }
#define T template
#define CL class

typedef unsigned uint;
typedef long long LL;
typedef unsigned long long ULL;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<LL> VLL;
typedef pair<int, int> PII;
typedef vector<PII> VPII;
typedef vector<VPII> VVPII;
typedef pair<LL, LL> PLLLL;
typedef vector<PLLLL> VPLLLL;
typedef vector<bool> VB;
typedef vector<char> VC;
typedef vector<string> VS;

T<CL A>
inline A abs(const A& a) { return a < A() ? -a : a; }

T<CL A, CL B>
inline void mini(A& a, const B& b) {
	if (b < a)
		a = b;
}

T<CL A, CL B>
inline void maxi(A& a, const B& b) {
	if (b > a)
		a = b;
}

T<CL Iter>
ostream& __out(ostream& os, Iter a, Iter b, const string& s = ", ");

T<CL A, CL B>
O(pair<A, B>);

T<CL A>
OO(vector<A>)

T<CL A>
OO(deque<A>)

T<CL A>
OO(list<A>)

T<CL A, CL B>
OO(set<A, B>)

T<CL A, CL B, CL C>
OO(map<A, B, C>)

T<CL A, CL B>
OO(multiset<A, B>)

T<CL A, CL B, CL C>
OO(multimap<A, B, C>)

T<CL Iter>
ostream& __out(ostream& os, Iter a, Iter b, const string& s) {
	os << "{";
	if (a != b) {
		os << *a;
		while (++a != b)
			os << s << *a;
	}
	return os << "}";
}

T<CL Iter>
ostream& __dump(ostream& os, Iter a, Iter b) {
	os << "{\n";
	Iter beg = a;
	while (a != b) {
		os << " " << a - beg << ": " << *a << "\n";
		++a;
	}
	return os << "}";
}

T<CL A, CL B>
O(pair<A, B>) {
	return os << "(" << x.ST << ", " << x.ND << ")";
}

CL Input {
	static const int BUFF_SIZE = 1 << 16;
	unsigned char buff[BUFF_SIZE], *pos, *end;

	void grabBuffer() {
		pos = buff;
		end = buff + read(0, buff, BUFF_SIZE);
	}

public:
	Input() : pos(buff), end(buff) {}

	int peek() {
		if (pos == end)
			grabBuffer();
		return pos != end ? *pos : -1;
	}

	int getChar() {
		if (pos == end)
			grabBuffer();
		return pos != end ? *pos++ : -1;
	}

	void skipWhiteSpaces() {
		while (isspace(peek()))
			++pos;
	}

	T<CL A>
	A get();

	void operator()(char* s) {
		skipWhiteSpaces();
		while (!isspace(peek()))
			*s++ = *pos++;
		*s = '\0';
	}

	T<CL A>
	void operator()(A& x) { x = get<A>(); }

	T<CL A, CL B>
	void operator()(A& a, B& b) { operator()(a), operator()(b); }

	T<CL A, CL B, CL C>
	void operator()(A& a, B& b, C& c) { operator()(a, b), operator()(c); }

	T<CL A, CL B, CL C, CL D>
	void operator()(A& a, B& b, C& c, D& d) {
		operator()(a, b, c);
		operator()(d);
	}

	T<CL A, CL B, CL C, CL D, CL E>
	void operator()(A& a, B& b, C& c, D& d, E& e) {
		operator()(a, b, c, d);
		operator()(e);
	}

	T<CL A, CL B, CL C, CL D, CL E, CL F>
	void operator()(A& a, B& b, C& c, D& d, E& e, F& f) {
		operator()(a, b, c, d, e);
		operator()(f);
	}

	Input& operator>>(char *s) {
		operator()(s);
		return *this;
	}

	T<CL A>
	Input& operator>>(A& a) {
		operator()(a);
		return *this;
	}
} fin;

T<> char Input::get<char>() {
	skipWhiteSpaces();
	return *pos++;
}

T<> uint Input::get<uint>() {
	skipWhiteSpaces();
	uint x = 0;
	while (isdigit(peek()))
		x = x * 10 + *pos++ - '0';
	return x;
}

T<> int Input::get<int>() {
	skipWhiteSpaces();
	return peek() == '-' ? (++pos, -get<uint>()) : get<uint>();
}

T<> ULL Input::get<ULL>() {
	skipWhiteSpaces();
	ULL x = 0;
	while (isdigit(peek()))
		x = x * 10 + *pos++ - '0';
	return x;
}

T<> LL Input::get<LL>() {
	skipWhiteSpaces();
	return peek() == '-' ? (++pos, -get<ULL>()) : get<ULL>();
}

T<> string Input::get<string>() {
	skipWhiteSpaces();
	string x;
	while (!isspace(peek()))
		x += *pos++;
	return x;
}

int ceil2(int x) { return 1 << (sizeof(x) * 8 -__builtin_clz(x - 1)); }

#undef T
#undef CL
#ifdef DEBUG
# define D(...) __VA_ARGS__
# define E(...) eprintf(__VA_ARGS__)
# define OUT(a,b) cerr << #a ":", __out(cerr, a, b), E("\n")
# define DUMP(x) cerr << #x ":", __dump(cerr, ALL(x)), E("\n")
# define LOG(x) cerr << #x ": " << (x)
# define LOG2(x, y) cerr << x << ": " << (y)
# define LOGN(x) LOG(x) << endl
# define LOGN2(x, y) LOG2(x, y) << endl
#else
# define D(...)
# define E(...)
# define OUT(...)
# define DUMP(...)
# define LOG(...)
# define LOG2(...)
# define LOGN(...)
# define LOGN2(...)
#endif
/// End of templates

int n;
VVI G;
VI cycle; // a <- b <- ... <- x (<- a) // without a at the end
#ifdef DEBUG
VI vis;
#else
VC vis;
#endif
VI id; // id non-negative only for cycle nodes
// VPII mosts; // most left node id, right most node id (id is cycle node id)

// -1 - cycle found, 0 - cycle not found, > 0 number of node on which cease
// adding nodes to cycle (cycle found)
int find_cycle(int x) {
	// D(static int k = 0;++k;)
	// LOGN2(string(k, ' '), x);
	vis[x] = 1;
	int tmp;
	FOREACH (i, G[x])
		if (vis[*i] == 1) {
			cycle.PB(x);
			vis[x] = 2;
			return *i;

		} else if (vis[*i] == 0 && (tmp = find_cycle(*i)) != 0) {
			vis[x] = 2;
			if (tmp == -1)
				return -1;

			cycle.PB(x);
			return x == tmp ? -1 : tmp;
		}

	// D(--k);
	vis[x] = 2;
	return 0;
}

void remove_edge(int a, int b) {
	FOREACH (i, G[a])
		if (*i == b) {
			swap(*i, G[a].back());
			G[a].pop_back();
			break;
		}
}

VPII most;
PII get_most(int x) {
	if (most[x] != PII(-1, -1))
		return most[x];

	PII tmp(n + 1, -1);
	FOREACH (i, G[x]) {
		PII ggg;
		if (id[*i] != -1)
			ggg = PII(id[*i], id[*i]);
		else
			ggg = get_most(*i);

		mini(tmp.ST, ggg.ST);
		maxi(tmp.ND, ggg.ND);
	}
	return most[x] = tmp;
}

VB attainable;
void mark(int x, int k) {
	if (id[x] > k)
		return;
	FOREACH (i, G[x])
		if (!attainable[*i]) {
			attainable[*i] = true;
			mark(*i, k);
		}
}

int main() {
	int m, a, b;
	fin >> n >> m;
	G.resize(n + 1);
	vis.resize(n + 1);
	id.resize(n + 1, -1);
	REP (i, m) {
		fin >> a >> b;
		G[a].PB(b);
	}

	// Find cycle
	FOR (i, 1, n + 1)
		if (vis[i] == 0 && find_cycle(i))
			break;

	if (cycle.empty()) {
		puts("NIE\n");
		return 0;
	}
	LOGN(cycle);

	// Number cycle nodes
	REP (i, SZ(cycle))
		id[cycle[i]] = i;
	LOGN(id);

	// Check if there is separate cycle from the found
	REP (i, n + 1)
		vis[i] = (id[i] == -1 ? 0 : 2);
	LOGN(vis);
	FOR (i, 1, n + 1)
		if (vis[i] == 0 && find_cycle(i) != 0) {
			puts("0\n");
			return 0;
		}

	// Remove edges of cycle
	remove_edge(cycle[0], cycle.back());
	FOR (i, 1, SZ(cycle))
		remove_edge(cycle[i], cycle[i - 1]);
	DUMP(G);

	// Process other cycles
	VI good(SZ(cycle) + 1); // if prefix sum of node i is 0 then i belongs to result
	most.resize(n + 1, PII(-1, -1));
	attainable.resize(n + 1);
	FORD (i, SZ(cycle) - 1, 0) {
		PII best = get_most(cycle[i]); // left most node id, right most node id

		E("====================\n");
		LOGN(i);
		LOGN(cycle[i]);
		LOGN(best);

		// Found cycle
		if (best.ND >= i) {
			// Mark all nodes attainable from <1, i> nodes
			REP (j, i + 1)
				if (!attainable[cycle[j]])
					mark(cycle[j], i);
			FOR (j, i, best.ND)
				if (attainable[cycle[j]]) {
					best.ND = j;
					break;
				}

			LOGN(attainable);
			D(
				REP (j, SZ(cycle))
					E(" %i", (int)attainable[cycle[j]]);
				E("\n");
			)
			LOGN(best.ND);
			// Check if there is a cycle beginning in a, and ending in b (a, b are in cycle) and id[a] < id[b] < i
			REP (j, n + 1)
				vis[j] = (id[j] >= i ? 2 : 0);
			// Restore some of cycle edges
			FOR (j, 1, i)
				G[cycle[j]].PB(cycle[j - 1]);
			LOGN(vis);
			REP (j, i)
				if (vis[cycle[j]] == 0 && find_cycle(cycle[j])) {
					// Found such a cycle
					puts("0\n");
					return 0;
				}
			LOGN(vis);

			// Erase everything not from <i, best.ND>
			++good[0];
			--good[i];
			++good[best.ND + 1];
			--good[SZ(cycle)];
			break;
		}

		assert(best.ST != i);
		// Found alternative way to move forward
		if (best.ST < i) {
			++good[best.ST + 1];
			--good[i];
		}
	}
	LOGN(most);
	LOGN(good);
	// Print result
	VI res;
	REP (i, SZ(cycle)) {
		good[i + 1] += good[i];
		if (good[i] == 0)
			res.PB(cycle[i]);
	}

	sort(ALL(res));
	printf("%i\n", SZ(res));
	REP (i, SZ(res))
		printf(i ? " %i" : "%i", res[i]);
	puts("\n");
	return 0;
}
/*
4 5
1 2
2 3
3 1
3 4
4 2

3 2
1 2
2 3

5 7
1 4
4 5
5 3
3 2
2 1
3 4
5 1

5 8
1 4
4 5
5 3
3 2
2 1
3 4
5 1
2 3

4 5
1 2
2 3
3 1
2 4
4 2

*/