#include <bits/stdc++.h>
using namespace std;
#define int long long
#define arr array
#define vec vector
#define pii pair<int, int>
#define fir first
#define sec second
const int N = 5e5 + 5;
int n, m;
arr<vec<int>, N> in, out;
arr<bool, N> tp_vs;
vec<int> tp;
void tp_dfs(int u) {
tp_vs[u] = true;
for (int v : out[u])
if (!tp_vs[v]) tp_dfs(v);
tp.push_back(u);
}
int nm_sccs;
arr<int, N> scc;
arr<vec<int>, N> scc_nds;
void scc_dfs(int u) {
scc[u] = nm_sccs, scc_nds[nm_sccs].push_back(u);
for (int v : in[u])
if (!scc[v]) scc_dfs(v);
}
void prcmp() {
for (int u = 1; u <= n; u++)
if (!tp_vs[u]) tp_dfs(u);
reverse(tp.begin(), tp.end());
for (int u : tp)
if (!scc[u]) nm_sccs++, scc_dfs(u);
// for (int i = 1; i <= nm_sccs; i++) {
// cout << i << ": ";
// for (int u : scc_nds[i]) cout << u << " ";
// cout << endl;
// }
}
arr<int, N> typ, cnt;
set<pii> ord;
void intl(int id) {
ord.clear();
for (int u : scc_nds[id]) {
if (typ[u] != 0) continue;
for (int v : out[u])
cnt[u] += (typ[v] == 0);
ord.insert({cnt[u], u});
}
}
void rmv(int u) {
if (!ord.count({cnt[u], u})) return;
ord.erase({cnt[u], u});
for (int v : in[u]) {
if (!ord.count({cnt[v], v})) continue;
ord.erase({cnt[v], v});
cnt[v]--;
ord.insert({cnt[v], v});
}
}
arr<int, N> cl;
void cl_dfs(int u) {
for (int v : out[u]) {
if (cl[v] || scc[v] != scc[u]) continue;
cl[v] = (cl[u] == 1) ? 2 : 1;
cl_dfs(v);
}
}
void slv(int id) {
intl(id);
while (ord.size() && ord.begin()->fir == 0) {
int u = ord.begin()->sec;
ord.erase(ord.begin());
typ[u] = 1, rmv(u);
for (int v : in[u])
typ[v] = 2, rmv(v);
}
cl[scc_nds[id][0]] = 1, cl_dfs(scc_nds[id][0]);
for (int u : scc_nds[id]) {
if (typ[u] != 0) continue;
if (cl[u] != 1) continue;
typ[u] = 1;
for (int v : in[u]) typ[v] = 2;
}
}
void cmp() {
for (int i = nm_sccs; i >= 1; i--)
slv(i);
vec<int> ans;
for (int u = 1; u <= n; u++) {
assert(typ[u] != 0);
if (typ[u] == 1) ans.push_back(u);
}
cout << ans.size() << endl;
for (int u : ans) cout << u << " ";
cout << endl;
}
signed main() {
// freopen("a.in", "r", stdin);
cin >> n >> m;
for (int i = 1; i <= m; i++) {
int u, v; cin >> u >> v;
out[u].push_back(v), in[v].push_back(u);
}
prcmp();
cmp();
}
