// #pragma optimize ("g",on)
// #pragma GCC optimize ("inline")
// #pragma GCC optimize ("Ofast")
// #pragma GCC optimize ("unroll-loops")
// #pragma GCC optimize ("03")
#include <bits/stdc++.h>
#define pb push_back
#define SS ios_base::sync_with_stdio(0);cin.tie(nullptr);cout.tie(nullptr);
// #define int long long
#define all(v) v.begin(),v.end()
using namespace std;
mt19937 rnd(chrono::steady_clock::now().time_since_epoch().count());
const int N = 5e5 + 17, MX = 1e7 + 1, inf = 1e9, mod = 998244353;
int n, m;
vector<int> g[N];
int col[N], used[N];
vector<int> v;
void dfs(int v, int p){
used[v] = 1;
for(int to : g[v]){
if(used[to]) continue;
col[to] = col[v] ^ 1;
dfs(to, v);
}
}
void solve(){
cin >> n >> m;
for(int i = 1; i <= m; i++){
int a, b;
cin >> a >> b;
g[b].push_back(a);
}
for(int i = 1; i <= n; i++){
col[i] = -1;
}
for(int i = 1; i <= n; i++){
if(used[i]) continue;
col[i] = 0;
dfs(i, i);
}
for(int i = 1; i <= n; i++){
if(!col[i]){
v.push_back(i);
}
}
cout << v.size() << '\n';
for(int i : v) cout << i << ' ';
}
signed main(){
SS
// freopen("trains.in", "r", stdin);
// freopen("trains.out", "w", stdout);
int t = 1;
// cin >> t;
while(t--){
solve();
}
}
