#include <bits/stdc++.h>
using namespace std;
#ifdef ONPC
#include "debug.h"
#else
#define dbg(...)
#endif
#define ll long long
#define int long long
#define ld long double
#define pi pair<int, int>
#define sz(a) ((int)(a.size()))
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define sqr(n) ((n) * (n))
#define divup(a, b) (((a) + (b)-1) / (b))
#define popcount(n) __builtin_popcountll(n)
#define clz(n) __builtin_clzll(n)
#define Fixed(a) cout << fixed << setprecision(12) << a;
template <class T> bool chmin(T& a, const T& b) { return b < a ? a = b, 1 : 0; }
template <class T> bool chmax(T& a, const T& b) { return b > a ? a = b, 1 : 0; }
const int mod = 998244353; // 998244353 1e9 + 7
const ll inf = (ll)(1e18) + 7;
const ld eps = 1e-9;
const int B = 32;
const int N = 1000 + 3;
const int logn = 20;
const int maxn = 5e5 + 7;
/////////////////////////solve/////////////////////////
vector<int> g[maxn], gt[maxn], gc[maxn];
vector<vector<int>> t;
int comp[maxn];
vector<int> top;
bool used[maxn];
int col[maxn];
void dfs(int v) {
used[v] = 1;
for (auto u : g[v]) {
if (!used[u]) dfs(u);
}
top.push_back(v);
}
void dfs2(int v, int k) {
used[v] = 1;
comp[v] = k;
for (auto u : gt[v]) {
if (!used[u]) dfs2(u, k);
}
}
void dfs3(int v) {
used[v] = 1;
for (auto u : gc[v]) {
if (!used[u]) dfs3(u);
}
for (auto v : t[v]) {
if (col[v]) continue;
bool red = 1;
for (auto u : g[v]) {
if (comp[v] == comp[u]) {
if (col[u] == 1) red = 0;
}
}
for (auto u : gt[v]) {
if (comp[v] == comp[u]) {
if (col[u] == 1) red = 0;
}
}
if (red) {
col[v] = 1;
for (auto u : g[v]) {
col[u] = 2;
}
for (auto u : gt[v]) {
col[u] = 2;
}
}
else col[v] = 2;
}
}
void solve() {
int n, m;
cin >> n >> m;
for (int i = 0; i < m; ++i) {
int v, u;
cin >> v >> u;
--v; --u;
g[v].push_back(u);
gt[u].push_back(v);
}
for (int i = 0; i < n; ++i) {
if (!used[i]) dfs(i);
}
reverse(all(top));
memset(used, 0, sizeof(used));
int k = 0;
for (auto v : top) {
if (!used[v]) dfs2(v, k++);
}
t.resize(k);
for (int v = 0; v < n; ++v) {
t[comp[v]].push_back(v);
for (auto u : g[v]) {
if (comp[u] != comp[v]) gc[v].push_back(u);
}
}
memset(used, 0, sizeof(used));
for (int v = 0; v < k; ++v) {
// dbg(v, t[v], comp[v]);
if (!used[v]) {
dfs3(v);
}
}
int ans = 0;
for (int i = 0; i < n; ++i) {
if (col[i] == 1) ++ans;
}
cout << ans << "\n";
for (int i = 0; i < n; ++i) {
if (col[i] == 1) cout << i + 1 << " ";
}
}
signed main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
#ifdef ONPC
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
freopen("error.txt", "w", stderr);
#endif
solve();
}
