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#include <bits/stdc++.h>
using namespace std;

int d, m, n, s, t, A[500001], B[500001], C[500001], D[500001], E[500001];
queue<int> Q;
vector<int> K[500001], K2[500001];

void wyznacz_skladowa(int a) {
	A[a] = B[a] = ++t;
	C[a] = 1;
	D[++d] = a;
	for (auto b: K[a]) {
		if (!A[b]) {
			wyznacz_skladowa(b);
			B[a] = min(B[a], B[b]);
		} else if (C[b]) {
			B[a] = min(B[a], A[b]);
		}
	}
	if (A[a] == B[a]) {
		if (D[d] == a) {
			E[a] = 1;
		} else {
			++s;
		}
		while (D[d] != a) {
			C[D[d]] = 0;
			--d;
		}
		C[a] = 0;
		--d;
	}
}

bool jest_cykl_po_usunieciu(int a) {
	memcpy(B, A, (n + 1) * sizeof(int));
	for (auto b: K2[a]) {
		--B[b];
	}
	B[a] = 0;
	E[a] = 1;
	for (int i = 1; i <= n; ++i) {
		if (!E[i] && !B[i]) {
			Q.push(i);
		}
	}
	while (!Q.empty()) {
		int i = Q.front();
		Q.pop();
		for (auto j: K2[i]) {
			if (E[j]) {
				continue;
			}
			if (--B[j] == 0) {
				Q.push(j);
			}
		}
	}
	E[a] = 0;
	for (int i = 1; i <= n; ++i) {
		if (B[i]) {
			return true;
		}
	}
	return false;
}

int main() {
	int a, b;
	scanf("%d %d", &n, &m);
	while (m--) {
		scanf("%d %d", &a, &b);
		K[a].push_back(b);
	}
	d = s = t = 0;
	for (int i = 1; i <= n; ++i) {
		if (!A[i]) {
			wyznacz_skladowa(i);
		}
	}
	if (!s) {
		puts("NIE");
		return 0;
	}
	if (s > 1) {
		puts("0");
		puts("");
		return 0;
	}
	memset(A, 0, (n + 1) * sizeof(int));
	for (int i = 1; i <= n; ++i) {
		if (E[i]) {
			continue;
		}
		for (auto j: K[i]) {
			if (E[j]) {
				continue;
			}
			++A[j];
			K2[i].push_back(j);
		}
	}
	d = 0;
	for (int i = 1; i <= n; ++i) {
		if (E[i]) {
			continue;
		}
		if (!jest_cykl_po_usunieciu(i)) {
			D[d++] = i;
		}
	}
	printf("%d\n", d);
	for (int i = 0; i < d; ++i) {
		printf("%d ", D[i]);
	}
	puts("");
	return 0;
}