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#include <cstdio>
#include <iostream>
#include <vector>
#include <list>
#include <string>
#include <set>
#include <map>
#include <algorithm>
#include <utility>
#include <cmath>
#include <queue>
#include <stack>
#include <cassert>
#include <cstring>
#include <climits>
#include <functional>
#include <unordered_set>
#include <unordered_map>
#define VAR(i,v) __typeof(v) i = (v)
#define SIZE(x) ((int)(x).size())
#define ALL(x) (x).begin(), (x).end()
#define REP(i,b) for(int i=0; i<(b); ++i)
#define FOR(i,a,b) for(int i=(a); i<=(b); ++i)
#define FORD(i,a,b) for(int i=(a); i>=(b); --i)
#define FOREACH(i,c) for(VAR(i,(c).begin()); i != (c).end(); ++i)
#define PB push_back
#define MP make_pair
#define ST first
#define ND second
#define NL printf("\n")

using namespace std;
typedef pair<int,int> PII;
typedef vector<int> VI;
typedef long long LL;

const int INF = 2147483640;
const int MAXN = 1000005;
const bool DEBUG = 0;
multiset<int> g[MAXN], h[MAXN];
multiset<int> path;
int in[MAXN];
vector<VI> cycles;
int hm[MAXN];
VI res;
int n, m;
bool brk = 0;

void find_cycle(int u, int b) {
	if(DEBUG) printf("find_cycle(%d, %d)\n", u, b);
	while(!h[u].empty()) {
		int v = *h[u].begin();
		h[u].erase(h[u].begin());
		res.PB(v);
		if(v == b) {
			cycles.PB(res);
			res.clear();
			brk = 1;
			return;
		}
		find_cycle(v,b);
		if(brk) return;
		if(!res.empty()) res.erase(--res.end());
	}
	
}

int main() {
	scanf("%d %d", &n, &m);
	REP(i,m) {
		int a, b;
		scanf("%d %d", &a, &b);
		g[a].insert(b);
	}

	FOR(i,1,n) {
		FOR(i,1,n) h[i] = g[i];
		while(!h[i].empty()) {
			brk = 0;
			res.PB(i);
			find_cycle(i,i);
			res.clear();
		}
	}

	if(!cycles.empty()) {
		int p = 0;
		for(VI X: cycles) {
			if(X.front() != X.back()) continue;
			p++;
			sort(ALL(X));
			X.erase(unique(ALL(X)), X.end());
			for(int x: X) hm[x]++;
		}
		VI answer;
		FOR(i,1,n) if(hm[i] == p) answer.PB(i);
		printf("%d\n", SIZE(answer));
		for(int x: answer) printf("%d ", x);
		NL;
	} else {
		printf("NIE\n");
	}

	if(DEBUG) {
		for(VI X: cycles) {
			for(int x: X) printf("%d ", x);
			NL;
		}
	}

	return 0;
}