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#include<cstdio>
#include<cstdlib>
#include<cmath>
#include<cstring>
#include<cassert>
#include<iostream>
#include<algorithm>
#include<queue>
#include<stack>
#include<bitset>
#include<set>
#include<map>
#define REP(i,n) for(int i=0;i<(n);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(__typeof((c).begin())i=(c).begin();i!=(c).end();i++)
#define all(c) (c).begin(),(c).end()
#define scanf(...) scanf(__VA_ARGS__)?:0
#define eprintf(...) fprintf(stderr,__VA_ARGS__),fflush(stderr)
#define e1 first
#define e2 second
#define mp make_pair
#define pb push_back
#define eb emplace_back
using namespace std;
typedef long long ll;
typedef long double ld;
typedef unsigned int uint;
typedef unsigned long long ull;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
typedef pair<ll,int> pli;
typedef pair<int,ll> pil;
int n,m,x,a,b,k,kt,ilew1,skl[500001],ile[500001],ciag[500001],pos[500001],tree[500001],tree2[500001];
vector<int> v[500001],vr[500001],w;
bitset<500001> odw;
stack<int,vector<int> >stos;
void add(int *tree,int a,int b)
{
	a--;
	while (b) tree[b]++,b-=b&-b;
	while (a) tree[a]--,a-=a&-a;
}
int query(int *tree,int a)
{
	int ret=0;
	while (a<=ile[kt]) ret+=tree[a],a+=a&-a;
	return ret;
}
void dfswrzuc(int a)
{
	odw[a]=true;
	foreach(it,v[a]) if (!odw[*it]) dfswrzuc(*it);
	stos.push(a);
}
void dfspodziel(int a)
{
	odw[a]=true;
	skl[a]=k;
	ile[k]++;
	foreach(it,vr[a]) if (!odw[*it]) dfspodziel(*it);
}
void dfssort(int a)
{
	odw[a]=true;
	foreach(it,v[a]) if (!odw[*it] && skl[*it]==kt) dfssort(*it);
	ciag[++x]=a;
	pos[a]=x;
}
int main()
{
	scanf("%d%d",&n,&m);
	REP(i,m) scanf("%d%d",&a,&b),v[a].pb(b),vr[b].pb(a);
	FOR(i,1,n) if (!odw[i]) dfswrzuc(i);
	odw.reset();
	while (!stos.empty())
	{
		if (!odw[stos.top()]) k=stos.top(),dfspodziel(stos.top());
		stos.pop();
	}
	FOR(i,1,n) if (ile[i]>1) ilew1++,kt=i;
	if (!ilew1) puts("NIE"),exit(0);
	if (ilew1>1) puts("0\n"),exit(0);
	odw.reset();
	FOR(i,1,n) if (skl[i]==kt && !odw[i]) dfssort(i);
	assert(x==ile[kt]);
	reverse(ciag+1,ciag+ile[kt]+1);
	FOR(i,1,n) pos[i]=ile[kt]-pos[i]+1;
	FOR(i,1,n) if (skl[i]==kt) foreach(it,v[i]) if (skl[*it]==kt)
	{
		if (pos[*it]>pos[i]) add(tree,pos[i]+1,pos[*it]-1);
		else add(tree2,1,pos[*it]-1),add(tree2,pos[i]+1,ile[kt]);
	}
	FOR(i,1,ile[kt]) if (!(query(tree,i)|query(tree2,i))) w.pb(ciag[i]);
	printf("%lu\n",w.size());
	sort(all(w));
	foreach(it,w) printf("%d ",*it);
	puts("");
}