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#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
#include <map>
#include <set>
#include <queue>
//#include <stack>
#include <cmath>
#include <cassert>
#include <cstring>
#include <ext/numeric>
using namespace std ;
using namespace __gnu_cxx ;
typedef long long LL ;
typedef pair<int,int> PII ;
typedef vector<int> VI ;
const int INF = 1e9+100 ;
const LL INFLL = (LL)INF * (LL)INF ;
#define REP(i,n) for(i=0;i<(n);++i)
#define ALL(c) c.begin(),c.end()
#define VAR(v,i) __typeof(i) v=(i)
#define FOREACH(i,c) for(VAR(i,(c).begin());i!=(c).end();++i)
#define CLEAR(t) memset(t,0,sizeof(t))
#define PB push_back
#define MP make_pair
#define FI first
#define SE second

template<class T1,class T2> ostream& operator<<(ostream &s, const pair<T1,T2> &x) { s<< "(" << x.FI << "," << x.SE << ")"; return s;}
template<class T>           ostream& operator<<(ostream &s, const vector<T>   &t) { FOREACH(it, t) s << *it << " " ; return s ; }
template<class T>           ostream& operator<<(ostream &s, const set<T>      &t) { FOREACH(it, t) s << *it << " " ; return s ; }
template<class T1,class T2> ostream& operator<<(ostream &s, const map<T1, T2> &t) { FOREACH(it, t) s << *it << " " ; return s ; }

///////////////////////////////////////////

const int MAXN = 500100 ;
VI G[MAXN], revG[MAXN] ;
int used[MAXN] ;
bool revUsed[MAXN] ;

VI stack, cycle ;

bool DFS(int u) {	// zwraca true gdy natrafi na cykl
	stack.PB(u) ;
	used[u]=1 ;
	FOREACH(it, G[u]) {
		if(used[*it]==1) {	// mamy cykl
			do {
				cycle.PB(stack.back()) ;
				stack.pop_back() ;
			} while(cycle.back()!=(*it)) ;
			reverse(ALL(cycle)) ;
			return true ;
		}
		if(used[*it]==0) {
			if(DFS(*it)) return true ;
		}
	}
	used[u]=2 ;
	stack.pop_back() ;
	return false ;
}

bool findCycle(int n, const VI &forbidden) {
	CLEAR(used) ;
	FOREACH(it, forbidden)
		used[*it]=2 ;
	cycle.clear() ;
	stack.clear() ;
	for(int i=1 ; i<=n ; i++)
		if(!used[i])
			if(DFS(i)) return true ;
	return false ;
}

//////////////////////////////////////////////////

int posInCycle[MAXN] ;

int goForward(int u) {
	int ret=-1 ;
	used[u] = true ;
	FOREACH(it, G[u]) {
		ret = max(ret, posInCycle[*it]) ;
		if(!used[*it] && posInCycle[*it]==-1)
			ret = max(ret, goForward(*it)) ;
	}
	return ret ;
}

void goBackward(int u) {
	revUsed[u] = true ;
	FOREACH(it, revG[u])
		if(!revUsed[*it] && posInCycle[*it]==-1)
			goBackward(*it) ;
}

int main()
{
	ios_base::sync_with_stdio(0) ;
	int n, m, i, a, b ;
	cin >> n >> m ;
	while(m--) {
		cin >> a >> b ;
		G[a].PB(b) ;
		revG[b].PB(a) ;
	}
	
	if(!findCycle(n, VI())) {
		cout << "NIE" << endl ;
		return 0 ;
	}
	VI mainCycle = cycle ;
//	cout << "mainCycle : " << mainCycle << endl ;
	if(findCycle(n, mainCycle)) {
//		cout << "jest inny cykl: " << cycle << endl ;
		cout << 0 << endl << endl ;
		return 0 ;
	}
	
	memset(posInCycle, -1, sizeof(posInCycle)) ;
	REP(i, mainCycle.size())
		posInCycle[mainCycle[i]] = i ;
	
	CLEAR(used) ;
	CLEAR(revUsed) ;
	
	vector<bool> isBad(mainCycle.size()) ; // czy niespelnia jeden z trzech warunkow
	
	int bestForward = -1 ;
	bool canGoBack=false ;
	REP(i, mainCycle.size()) {
//		cout << "i=" << i << ", u = " << cycle[i] << ", bestForward = " << bestForward << ", canGoBack = " << canGoBack << endl ;
		if(bestForward > i) isBad[i] = true ;
		if(canGoBack) isBad[i] = true ;
		
		bestForward = max(bestForward, goForward(cycle[i])) ;
		goBackward(cycle[i]) ;
		FOREACH(it, G[cycle[i]])
			if(revUsed[*it]) canGoBack=true ;
	}
//	cout << isBad << endl ;
	
	canGoBack=false ;
	CLEAR(used) ;
	for(i=mainCycle.size()-1 ; i>=0 ; i--) {
		if(canGoBack) isBad[i]=true ;
		
		goForward(cycle[i]) ;
		FOREACH(it, revG[cycle[i]])
			if(used[*it]) canGoBack=true ;
	}
//	cout << isBad << endl ;
	
	VI ret ;
	REP(i, isBad.size())
		if(!isBad[i])
			ret.PB(mainCycle[i]) ;
	sort(ALL(ret)) ;
	cout << ret.size() << endl << ret << endl ;
}