#include <bits/stdc++.h>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>
using namespace std;
using namespace __gnu_pbds;
#if defined(__GNUC__) || defined(__clang__)
#pragma GCC optimize("Ofast,unroll-loops,inline-functions,no-stack-protector,03")
#pragma GCC target ("sse,sse2,sse3,ssse3,sse4,popcnt,abm,avx2,mmx,fma,avx,tune=native")
#pragma clang loop vectorize(enable)
#pragma clang loop interleave(enable)
#endif
#define int long long
#define sz(x) (int)x.size()
#define F first
#define S second
#define pb push_back
#define nl '\n'
#define o_set tree<int, null_type, less_equal<int>, rb_tree_tag, tree_order_statistics_node_update>
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
typedef pair<int, int> pii;
void file( string s = "" )
{
if( s.empty() )
return;
freopen((s + ".in").c_str(), "r", stdin);
freopen((s + ".out").c_str(), "w", stdout);
}
const int N = 2e5 + 2;
const int N1 = 1e6 + 2;
const int inf = 1e9 + 2;
const int INF = 1e18 + 2;
const int mod = 1e9 + 7;
int T = 1, n, m, us[N], dg[N];
vector< int > g[N], ans;
void dfs( int v, int cnt )
{
us[v] = cnt;
int ch = (cnt == 1);
for( int to: g[v] )
{
if( us[to] == cnt )
ch = 0;
if( !us[to] )
dfs(to, 3 - cnt);
}
if( ch )
ans.pb(v);
}
void solve()
{
cin >>n >>m;
for( int i = 1, u, v; i <= m; ++i )
{
cin >>u >>v;
g[v].pb(u);
++dg[u];
}
vector< pii > lv(n + 1);
for( int i = 1; i <= n; ++i )
lv[i] = {dg[i], i};
sort(lv.begin() + 1, lv.end());
for( int i = 1; i <= n; ++i )
if( !us[lv[i].S] )
dfs(lv[i].S, 1);
cout <<sz(ans) <<nl;
for( int u: ans )
cout <<u <<' ';
}
signed main()
{
file("");
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
// cin >>T;
while( T-- )
solve();
}
