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

long long current_cykle;
long long total_cykle;
long long cykle_przez[500001];
long long minusy[500001];
long long cykle_enter[500001];

vector<int> edges[500001];
vector<int> answers;

bool visited[500001];
bool onStack[500001];

void DFS(int x){
	onStack[x] = true;
	cykle_enter[x] = current_cykle;
	for(int i = 0; i < (int)edges[x].size(); ++i){
		int ng = edges[x][i];
		if(!visited[ng]){
			visited[ng] = true;
			DFS(ng);
		}else if(onStack[ng]){
			// CYKIEL
			current_cykle++;
			total_cykle++;
			minusy[ng]++;
		}
	}
	cykle_przez[x] = current_cykle - cykle_enter[x];
	current_cykle -= minusy[x];
	onStack[x] = false;
}

int main(){
	int n, m;
	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);
	}
	total_cykle = 0;
	current_cykle = 0;
	for(int i = 1; i <= n; ++i){
		minusy[i] = 0;
		visited[i] = false;
		onStack[i] = false;
	}
	
	for(int i = 1; i <= n; ++i){
		if(!visited[i]){
			visited[i] = true;
			DFS(i);
		}
	}
	
	if(total_cykle == 0){
		printf("NIE\n");
		return 0;
	}
	
	for(int i = 1; i <= n; ++i){
		if(cykle_przez[i] == total_cykle){
			answers.push_back(i);
		}
	}
	
	sort(answers.begin(), answers.end());
	
	printf("%d\n", (int)answers.size());
	for(int i = 0; i < (int)answers.size(); ++i){
		printf("%d ", answers[i]);
	}
	printf("\n");
	return 0;
}