#include <bits/stdc++.h>
using namespace std;
const int MAXN = 500005;
vector<int> adj[MAXN], radj[MAXN], ans;
int od[MAXN];
bool dd[MAXN];
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
int nodes, edges; cin >> nodes >> edges;
for (int i = 0; i < edges; i++){
int a, b; cin >> a >> b;
adj[a].push_back(b); radj[b].push_back(a);
od[a]++;
}
queue<int> q; int id = 1;
for (int i = 1; i <= nodes; i++)
if (od[i] == 0)
q.push(i);
while (id <= nodes){
while (!q.empty()){
int x = q.front(); q.pop();
ans.push_back(x); dd[x] = 1;
for (int px : radj[x]){
if (dd[px]) continue;
dd[px] = 1;
for (int ppx : radj[px]){
if (dd[ppx]) continue;
od[ppx]--;
if (od[ppx] == 0) q.push(ppx);
}
}
}
while (id <= nodes && dd[id]) id++;
if (id <= nodes){
ans.push_back(id); dd[id] = 1;
vector<int> vx;
for (int px : adj[id])
if (!dd[px]){
dd[px] = 1; vx.push_back(px);
}
for (int px : radj[id])
if (!dd[px]){
dd[px] = 1; vx.push_back(px);
}
for (int px : vx){
for (int ppx : radj[px]){
if (dd[ppx]) continue;
od[ppx]--;
if (od[ppx] == 0) q.push(ppx);
}
}
}
}
cout << ans.size() << '\n';
for (int x : ans) cout << x << ' ';
}
