#include <bits/stdc++.h>
#define pb push_back
using namespace std;
const int N = 5e5 + 5;
vector<int> adj[N*2], adjrev[N], vis(N), comp(N), col(N);
vector<int> nodes;
vector<vector<int> > group[2];
void dfs(int node){
vis[node] = 1;
for(auto itr: adj[node]){
if(!vis[itr]){
dfs(itr);
}
}
nodes.pb(node);
}
void dfs2(int node, int compcnt, int color){
vis[node] = 1;
comp[node] = compcnt;
col[node] = color;
group[col[node]][compcnt].pb(node);
for(auto itr: adjrev[node]){
if(!vis[itr]){
dfs2(itr, compcnt, (color^1));
}
}
}
int main(){
ios_base::sync_with_stdio(false);
cin.tie(0); cout.tie(0);
int n, m;
cin>>n>>m;
vector<pair<int, int> > edge(m);
for(int i = 0; i < m; i++){
int u, v;
cin>>u>>v;
edge[i] = {u, v};
adj[u].pb(v);
adjrev[v].pb(u);
}
for(int i = 1; i <= n; i++){
if(!vis[i]){
dfs(i);
}
}
reverse(nodes.begin(), nodes.end());
for(int i = 0; i <= n; i++) vis[i] = 0;
vector<int> v;
int compcnt = 0;
for(auto itr: nodes){
if(!vis[itr]){
group[0].pb(v);
group[1].pb(v);
dfs2(itr, compcnt, 0);
compcnt++;
}
}
for(int i = 1; i <= n; i++){
adj[i].clear();
} // memory israf etme
for(int i = 0; i < m; i++){
int fnode = comp[edge[i].first] * 2 + col[edge[i].first];
int snode = comp[edge[i].second] * 2 + col[edge[i].second];
adj[fnode].pb(snode);
}
vector<int> ans;
vector<int> take(N*2, 0); // 0 = ?, 1 aldık, -1 alamayız
take[(compcnt - 1) * 2 + 1] = 1; // son sccnin 1 ini almayı dene
for(auto itr: group[1][compcnt-1]) ans.pb(itr);
take[(compcnt - 1) * 2] = -1;
for(int i = compcnt-2; i >= 0; i--){
int node0 = i*2, node1 = i*2+1;
for(auto itr: adj[node0]){
if(take[itr] == 1) take[node0] = -1;
}
for(auto itr: adj[node1]){
if(take[itr] == 1) take[node1] = -1;
}
if(take[node0] == -1 && take[node1] == -1) continue;
if(take[node0] == -1){
take[node1] = 1;
for(auto itr: group[1][i]) ans.pb(itr);
}
else{
take[node0] = 1;
for(auto itr: group[0][i]) ans.pb(itr);
}
}
bool check = 1;
vector<int> inans(n+1), hehas(n+1);
for(auto itr: ans) inans[itr] = 1;
for(int i = 0; i < m; i++){
if(inans[edge[i].first] && inans[edge[i].second]) check = 0;
if(!inans[edge[i].first] && inans[edge[i].second]) hehas[edge[i].first] = 1;
}
for(int i = 1; i <= n; i++){
if(inans[i] == 0 && hehas[i] == 0) check = 0;
}
if(ans.size() == 0) check = 0;
if(check){
cout<<ans.size()<<endl;
for(auto itr: ans){
cout<<itr<<" ";
}
cout<<endl;
}
else{
for(int i = 0; i < 2*N; i++) take[i] = 0;
ans.clear();
take[(compcnt - 1) * 2 + 1] = -1; // son sccnin 1 ini almayı dene
for(auto itr: group[0][compcnt-1]) ans.pb(itr);
take[(compcnt - 1) * 2] = 1;
for(int i = compcnt-2; i >= 0; i--){
int node0 = i*2, node1 = i*2+1;
for(auto itr: adj[node0]){
if(take[itr] == 1) take[node0] = -1;
}
for(auto itr: adj[node1]){
if(take[itr] == 1) take[node1] = -1;
}
if(take[node0] == -1 && take[node1] == -1) continue;
if(take[node0] == -1){
take[node1] = 1;
for(auto itr: group[1][i]) ans.pb(itr);
}
else{
take[node0] = 1;
for(auto itr: group[0][i]) ans.pb(itr);
}
}
cout<<ans.size()<<endl;
for(auto itr: ans){
cout<<itr<<" ";
}
cout<<endl;
}
return 0;
}
