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

const long long int VV=500*1000+7;
const long long int XX=VV+7;

vector<int> graf[VV];

int N, dek[VV], tab[VV], v, e, minskok[VV], minn, maxx, wyn, dokiedynie;
char kol[VV];
bool czy, wypisz[VV];

int lsas(int ii) {return (graf[ii].size());}
int isas(int ii, int iii) {return (graf[ii][iii]);}

void DFS1(int w)
{
	int j;
	
	dek[w] = XX + 1;
	
	for (j = 0; j < lsas(w); j++)
		if (dek[isas(w, j)] == XX)
			DFS1(isas(w, j));
	
	dek[w] = N;
	tab[N] = w;
	N--;
	return;	
}

int DFS2(int w)
{
	int i, j;
	int ret = XX;
	
	kol[w] = 's';
	for (j = 0; j < lsas(w); j++)
	{
		i = isas(w, j);
		if (kol[i] == 's')
		{
			minn = max(minn, dek[i]);
			maxx = min(maxx, dek[w]);
			ret = min(ret, dek[i]);
			czy = true;
		}
		else if (kol[i] == 'b')
		{
			minskok[i] = DFS2(i);
			ret = min(ret, minskok[i]);
		}
		else if (kol[i] == 'c')
			ret = min(ret, minskok[i]);
	}
	kol[w] = 'c';
	return ret;
}

int i, j, k;

int main()
{
ios_base::sync_with_stdio(0);

cin >> v >> e;

for (i = 0; i < e; i++)
{
	cin >> j >> k;
	graf[j].push_back(k);
}

N = v;
minn = 1;
maxx = v;
wyn = 0;
czy = false;
dokiedynie = 0;
for (i = 1; i < v + 1; i++)
{
	dek[i] = XX;
	kol[i] = 'b';
	minskok[i] = XX;
	wypisz[i] = false;
}

for (i = 1; i < v + 1; i++)
	if (dek[i] == XX)
		DFS1(i);
		
for (i = 1; i < v + 1; i++)
	if (kol[i] == 'b')
		minskok[i] = DFS2(i);

if (czy == false) cout << "NIE";
else if (minn > maxx) cout << 0 << "\n";
else
{
	for (i = 1; i < maxx + 1; i++)
	{
		if (i >= minn)
		{
			wypisz[tab[i]] = true;
			wyn++;
		}
		for (k = 0; k < lsas(tab[i]); k++)
		{
			j = isas(tab[i], k);
			if (dek[j] > i + 1)
				if (minskok[j] <= i)
					minn = max(minn, dek[j]);
		}
	}
	cout << wyn << "\n";
	for (i = 1; i < v + 1; i++)
		if (wypisz[i] == true) cout << i << " ";
}
/*
cout << "tab: \n";		
for (i = 1; i < v + 1; i++)
	cout << tab[i] << " ";
cout << "\n";
cout << "minskok: \n";		
for (i = 1; i < v + 1; i++)
	cout << minskok[i] << " ";
cout << "\n";
cout << "minn, maxx: " << minn << ", " << maxx << "\n";*/
return 0;
}