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#include<cstdio>
#include<algorithm>
#include<vector>
#include<set>
#include<map>
#include<queue>
#include<cmath>
#include<iostream>
#include<string>
using namespace std;
#define F first
#define S second
#define MP make_pair
#define PB push_back
#define LL long long
#define PII pair<int, int>
#define PLL pair<LL, LL>

int n, m, in[500005];
vector<int> V[500005], R;
queue<int> Q;

bool topo(int ind)
{
	int x=0;
	if(ind!=0)
		x++;
	for(int i=1; i<=n; i++)
	{
		if(i!=ind && in[i]==0)
		{
			Q.push(i);
			x++;
		}
	}
	while(!Q.empty())
	{
		int v=Q.front();
		Q.pop();
		for(int i=0; i<(int)V[v].size(); i++)
		{
			in[V[v][i]]--;
			if(V[v][i]!=ind && in[V[v][i]]==0)
			{
				Q.push(V[v][i]);
				x++;
			}
		}
	}
	return x==n;
}

int main()
{
	//ios_base::sync_with_stdio(0);
	scanf("%d%d", &n, &m);
	for(int i=0; i<m; i++)
	{
		int a, b;
		scanf("%d%d", &a, &b);
		V[a].PB(b);
		in[b]++;
	}
	if(topo(0))
	{
		printf("NIE\n");
		return 0;
	}
	for(int i=1; i<=n; i++)
	{
		for(int j=1; j<=n; j++)
			in[j]=0;
		for(int k=1; k<=n; k++)
			if(k!=i)
				for(int j=0; j<(int)V[k].size(); j++)
					in[V[k][j]]++;
		if(topo(i))
		{
			R.PB(i);
			//printf("wtf %d\n", i);
		}
	}
	printf("%d\n", (int)R.size());
	for(int i=0; i<(int)R.size(); i++)
		printf("%d ", R[i]);
	if(!R.empty())
		printf("\n");
	return 0;
}