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#include <cstdio>
#include <cstring>
#include <vector>
#include <algorithm>

int N,M;
bool cycles;
std::vector<int> edges[500100];
bool visited[500100];
int parents[500100][20];
int start[500100];
int finish[500100];

int top[500100];
int expected[500100];
int cycle_begin[500100];
int cycle_end[500100];
int skip_begin[500100];
int skip_end[500100];

void searchParent(int v) {
	static int ts = 0;
	visited[v] = true;
	start[v] = ++ts;

	for (int i=0; i<edges[v].size(); ++i) {
		int n = edges[v][i];
		if (!visited[n]) {
			parents[n][0] = v;
			searchParent(n);
		}
	}

	finish[v] = ++ts;
}

void buildParents() {
	int level = 1;
	int dist = 2;

	while (dist <= N) {
		for (int i=1; i<= N; ++i) {
			parents[i][level] = parents[parents[i][level-1]][level-1];
		}

		level++;
		dist <<= 1;
	}
}

//is a parent of b
bool isParent(int a, int b) {
	return start[a]<=start[b] && finish[a]>=finish[b];
}

int getCommonParent(int a, int b) {
	if (isParent(a,b)) return a;
	if (parents[a][0]==0) return 0;

	int l=0, r=19;
	while (l<r) { //find first parent
		int m = (l+r)/2;
		if (parents[a][m]==0 || isParent(parents[a][m], b)) r=m;
		else l=m+1;
	}

	if (l==0) return parents[a][0];
	else return getCommonParent(parents[a][l-1], b);
}

int dfs(int v) {
	int res = 0;
	visited[v] = true;

	for (int i=0; i<edges[v].size(); ++i) {
		int n = edges[v][i];

		if (!visited[n]) { //tree edge
			res += dfs(n);
			cycle_begin[v] += cycle_begin[n];
			cycle_begin[v] -= cycle_end[n];
			if (!top[v] || isParent(top[n],top[v])) top[v] = top[n];
		} else if (isParent(n,v)) { //back edge
			res++;
			cycles = true;
			cycle_begin[v] += 1;
			cycle_end[n] += 1;
			if (!top[v] || isParent(n,top[v])) top[v] = n;
		} else if (start[v] < start[n] && finish[v] > finish[n]) { //forward
			if (top[n] > 0 && start[top[n]] <= start[v]) {
				skip_begin[v] += 1;
				skip_end[n] += 1;
			}
		} else if (start[v] > start[n]) { //cross
			int parent = getCommonParent(v,n);
			if (parent && top[n] > 0 && start[top[n]] <= start[parent]) {
				skip_begin[parent] += 1;
				skip_end[n] += 1;
			}
		}
	}

	return res;
}

void _getResult(int v, int exp, int &skip, std::vector<int>& result) {
	visited[v] = true;
	skip -= skip_end[v];
	//printf("%d %d %d\n",v,cycle_begin[v],skip);

	if (cycle_begin[v]==exp && skip==0) result.push_back(v);

	skip += skip_begin[v];

	for (int i=0; i<edges[v].size(); ++i) {
		int n = edges[v][i];
		if (!visited[n]) {
			_getResult(n, exp, skip, result);
		}
	}
}

std::vector<int> getResult(int v) {
	std::vector<int> res;
	int skip = 0;
	_getResult(v, expected[v], skip, res);
	return res;
}

int main() {
	scanf("%d %d",&N,&M);
	for (int i=0; i<M; ++i) {
		int a,b;
		scanf("%d %d",&a,&b);
		edges[a].push_back(b);
	}

	memset(visited,0,sizeof(visited));
	memset(parents,0,sizeof(parents));
	memset(start,0,sizeof(start));
	memset(finish,0,sizeof(finish));
	memset(expected,0,sizeof(expected));
	memset(top,0,sizeof(top));
	memset(cycle_begin,0,sizeof(cycle_begin));
	memset(cycle_end,0,sizeof(cycle_end));
	memset(skip_begin,0,sizeof(skip_begin));
	memset(skip_end,0,sizeof(skip_end));

	//look for parent
	for (int i=1; i<=N; ++i) {
		if (!visited[i]) {
			searchParent(i);
		}
	}

	//build parents array
	buildParents();

	//detect cycles
	int components = 0;
	memset(visited,0,sizeof(visited));
	for (int i=1; i<=N; ++i) {
		if (!visited[i]) {
			cycles = false;
			expected[i] = dfs(i);
			if (cycles) ++components; 
		}
	}

	//result
	if (components==0) printf("NIE\n");
	else if (components>1) printf("0\n");
	else {
		std::vector<int> result;
		memset(visited,0,sizeof(visited));
		for (int i=1; i<=N && result.size()==0; ++i) {
			if (!visited[i] && expected[i]>0) {
				result = getResult(i);
			}
		}

		std::sort(result.begin(), result.end());
		printf("%d\n",(int)result.size());
		for (int i=0; i<result.size(); ++i) printf("%d ",result[i]);
	}

	return 0;
}