#include <bits/stdc++.h>
using namespace std;
#define int long long
const int INF = 1e18;
const int MOD = 1e9+7;
const int MAXN = 5e5;
int n, m;
vector<int> adj[MAXN+5];
int inDegree[MAXN+5];
int par[MAXN+5];
int dp[5][MAXN+5];
vector<pair<int, bool>> pre[5][MAXN+5];
void dfs(int cur){
dp[1][cur] += 1;
for(auto next : adj[cur]){
if(next == par[cur]) continue;
dfs(next);
dp[0][cur] += max(dp[0][next], dp[1][next]);
dp[1][cur] += dp[0][next];
if(dp[0][next] > dp[1][next]) pre[0][cur].push_back({next, 0});
else pre[0][cur].push_back({next, 1});
pre[1][cur].push_back({next, 0});
}
}
set<int> st;
void backtrack(pair<int, bool> yey){
auto [cur, stat] = yey;
// cerr << "bcktrk : " << cur << ' ' << stat << endl;
if(stat) st.insert(cur);
for(auto i : pre[stat][cur]) backtrack(i);
}
void solve(){
cin >> n >> m;
for(int i = 1; i <= m; i++){
int u, v; cin >> u >> v;
adj[u].push_back(v);
inDegree[v]++;
}
int ans = 0;
for(int i = 1; i <= n; i++){
if(inDegree[i] > 0) continue;
dfs(i);
}
for(int i = 1; i <= n; i++){
if(inDegree[i] > 0) continue;
// cerr << "masuk : " << i << endl;
if(dp[0][i] > dp[1][i]) backtrack({i, 0});
else backtrack({i, 1});
}
cout << st.size() << endl;
for(auto i : st) cout << i << ' ';
cout << endl;
// for(int i = 1; i <= n; i++){
// cerr << i << ' ' << 0 << " : ";
// for(auto j : pre[0][i]) cerr << j.first << ' ' << j.second << ' ';
// cerr << endl;
// cerr << i << ' ' << 1 << " : ";
// for(auto j : pre[1][i]) cerr << j.first << ' ' << j.second << ' ';
// cerr << endl;
// }
}
signed main(){
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
int tc = 1;
// cin >> tc;
while(tc--){
solve();
}
return 0;
}
/*
4 3
1 2
1 3
2 4
=> 2 {4, 1}
4 4
1 2
1 3
2 4
3 4
=> 2 {4, 1}
*/
