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#include <cstdio>
#include <vector>
#include <algorithm>
#include <cstdlib>
using namespace std;

const int N = 500001;
vector<int> g[N];
vector<int> rg[N];
vector<int> order;
bool vis[N];
vector<int> vec;

void dfs1(int v)
{
    vis[v] = true;
    for(int u: g[v])
	if(!vis[u]) dfs1(u);
    order.push_back(v);
}

void dfs2(int v)
{
    vis[v] = true;
    vec.push_back(v);
    for(int u: rg[v])
	if(!vis[u]) dfs2(u);
}

vector<int> scc(int n)
{
    for(int i = 1; i <= n; i++)
	if(!vis[i]) dfs1(i);
    for(int i = 1; i <= n; i++)
	vis[i] = false;
    vector<int> ans;
    reverse(order.begin(), order.end());
    for(int v: order)
	if(!vis[v])
	{
	    vec.clear();
	    dfs2(v);
	    if(vec.size() != 1)
		if(ans.empty()) ans = vec;
		else
		{
		    puts("0\n");
		    exit(0);
		}
	}
    if(ans.empty())
    {
	puts("NIE");
	exit(0);
    }
    return ans;
}

int k[N];
int sum[N];
int d[N];
int low[N];
int col[N];
int type[N];

int back = 0;
int dfs3(int v)
{
    col[v] = 1;
    for(int u: g[v])
	if(col[u] == 0)
	{
	    d[u] = d[v] + 1;
	    sum[v] += dfs3(u);
	}
	else if(col[u] == 1)
	{
	    back++;
	    sum[v]++;
	    k[u]++;
	    type[v] = 3;
	}
    col[v] = 2;
    return sum[v] - k[v];
}

void dfs4(int v)
{
    vis[v] = true;
    if(type[v] == 3)
    {
	low[v] = 1000000;
	return;
    }
    if(type[v] == 2)
	low[v] = d[v];
    for(int u: g[v])
	if(type[u] == 2)
	    low[v] = max(low[v], d[u]);
	else
	{
	    if(!vis[u])
		dfs4(u);
	    low[v] = max(low[v], low[u]);
	}
}

vector<int> ans;
void dfs5(int v, int l)
{
    if(type[v] == 2 && l <= d[v])
	ans.push_back(v);
    l = max(l, low[v]);
    vis[v] = true;
    for(int u: g[v])
	if(!vis[u])
	    dfs5(u, l);
}

int main()
{
    int n, m;
    scanf("%d %d", &n, &m);
    while(m--)
    {
	int a, b;
	scanf("%d %d", &a, &b);
	g[a].push_back(b);
	rg[b].push_back(a);
    }
    auto vec = scc(n);
    for(int v: vec)
	type[v] = 1;
    for(auto v: vec)
    {
	auto tmp = g[v];
	g[v].clear();
	for(int u: tmp)
	    if(type[u] == 1) g[v].push_back(u);
    }
    dfs3(vec[0]);
    for(int v: vec)
    {
	if(sum[v] == back) type[v] = 2;
	vis[v] = false;
    }
    for(int v: vec)
	if(!vis[v]) dfs4(v);
    for(int v: vec)
	vis[v] = false;
    dfs5(vec[0], 0);
    sort(ans.begin(), ans.end());
    printf("%d\n", ans.size());
    for(int u: ans) printf("%d ", u);
    puts("");
}