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#include <bits/stdc++.h>
using namespace std;

#define fru(j,n) for(int j=0; j<(n); ++j)
#define tr(it,v) for(auto it=(v).begin(); it!=(v).end(); ++it)
#define x first
#define y second
#define pb push_back
#define mp make_pair
#define ALL(G) (G).begin(),(G).end()

typedef long long ll;
typedef double D;
typedef pair<int,int> pii;
typedef vector<int> vi;

const int inft = 1000000009;
const int MAXN = 1000006;

vi V[MAXN];
int vis[MAXN];
int n,m,nr;

bool dfs(int a,int zab){
//	printf("a= %d, zab %d\n",a,zab);
	if(vis[a]==1)return true;
	if(vis[a]==2)return false;
	vis[a]=1;
	tr(it,V[a])if(*it!=zab && dfs(*it,zab))return true;
	vis[a]=2;
	return false;
}
bool check(int zab){
	fru(i,n)vis[i]=-1;
	fru(i,n)if(i!=zab && vis[i]==-1 && dfs(i,zab))return true;
	return false;
}
void solve() {
	scanf("%d%d",&n,&m);
	fru(i,m){
		int a,b;
		scanf("%d%d",&a,&b);a--;b--;
		V[a].pb(b);
	}
	//find max reachable cycle vertex from v
	vi ANS;

	if(!check(-1)){puts("NIE");return;}
	fru(i,n)if(!check(i))ANS.pb(i);
	printf("%d\n",ANS.size());
	tr(it,ANS)printf("%d ",*it+1);
}

int main() {
//	freopen("input.in", "r", stdin);
//	freopen("output.out", "w", stdout);
	int t=1;
//	scanf("%d",&t);
	fru(i,t) solve();
	return 0;
}