#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx,popcnt,sse4,abm")
#include <bits/stdc++.h>
using namespace std;
#ifdef WAIMAI
#define debug(HEHE...) cout << "[" << #HEHE << "] : ", dout(HEHE)
void dout() {cout << '\n';}
template<typename T, typename...U>
void dout(T t, U...u) {cout << t << (sizeof...(u) ? ", " : ""), dout(u...);}
#else
#define debug(...) 7122
#endif
#define ll long long
#define Waimai ios::sync_with_stdio(false), cin.tie(0)
#define FOR(x,a,b) for (int x = a, I = b; x <= I; x++)
#define pb emplace_back
#define F first
#define S second
const int SIZE = 5e5 + 5;
int n, m;
int ans, cl[SIZE];
vector<int> adj[SIZE], rev[SIZE];
int tid, sid, dfn[SIZE], low[SIZE], ist[SIZE], scc[SIZE];
vector<int> st, v[SIZE];
void dfs(int pos) {
dfn[pos] = low[pos] = ++tid;
st.pb(pos), ist[pos] = 1;
for (int np : adj[pos]) {
if (dfn[np] == 0) {
dfs(np);
low[pos] = min(low[pos], low[np]);
} else if (ist[np]) {
low[pos] = min(low[pos], dfn[np]);
}
}
if (dfn[pos] == low[pos]) {
sid++;
while (st.size()) {
int x = st.back();
st.pop_back();
ist[x] = 0;
scc[x] = sid;
v[sid].pb(x);
if (x == pos) break;
}
}
}
void solve() {
cin >> n >> m;
while (m--) {
int a, b;
cin >> a >> b;
adj[a].pb(b);
rev[b].pb(a);
}
FOR (i, 1, n) if (dfn[i] == 0) dfs(i);
FOR (i, 1, sid) {
for (int x : v[i]) {
for (int y : adj[x]) if (cl[y] == 2) {
cl[x] = 1;
break;
}
}
for (int x : v[i]) if (cl[x] == 0) {
vector<int> u;
queue<int> q;
cl[x] = -1, q.push(x);
while (q.size()) {
int pos = q.front();
q.pop();
u.pb(pos);
for (int np : adj[pos]) if (scc[np] == i && cl[np] == 0) {
cl[np] = -1;
q.push(np);
}
for (int np : rev[pos]) if (scc[np] == i && cl[np] == 0) {
cl[np] = -1;
q.push(np);
}
}
int s = 0;
for (int y : u) {
bool f = 1;
for (int z : adj[y]) if (cl[z] != 1) {
f = 0;
break;
}
if (f) {
s = y;
break;
}
}
if (s == 0) s = x;
cl[s] = 2, q.push(s);
while (q.size()) {
int pos = q.front();
q.pop();
for (int np : adj[pos]) if (cl[np] == -1) {
cl[np] = 3 - cl[pos];
q.push(np);
}
for (int np : rev[pos]) if (cl[np] == -1) {
cl[np] = 3 - cl[pos];
q.push(np);
}
}
}
}
FOR (i, 1, n) ans += (cl[i] == 2);
cout << ans << '\n';
FOR (i, 1, n) if (cl[i] == 2) cout << i << ' ';
cout << '\n';
}
int main() {
Waimai;
solve();
}
