#pragma GCC optimize("Ofast")
 
#include <bits/stdc++.h>   
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;   
using namespace std;
#define ll long long
#define ld long double
#define nline "\n"
#define f first
#define s second
#define sz(x) (ll)x.size()
#define vl vector<ll>
const ll INF_MUL=1e13;
const ll INF_ADD=1e18;
#define all(x) x.begin(),x.end()
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
typedef tree<ll, null_type, less<ll>, rb_tree_tag, tree_order_statistics_node_update> ordered_set;
//--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------     
const ll MOD=998244353;
const ll MAX=1001000;
vector<pair<ll,ll>> adj[MAX];
ll val[MAX],valid,n,m,g=0;
void dfs(ll cur){
  if(!valid){
    return;
  }
  for(auto edge:adj[cur]){
    if(!valid){
      return;
    }
    ll node=edge.f,weight=edge.s;
    ll new_val=val[cur]+weight;
    if(g!=0){
      new_val%=g;
      new_val+=g;
      new_val%=g;
    }
    if(val[node]==-1){
      val[node]=new_val;
      dfs(node);
    }
    else if(new_val!=val[node]){
      g=__gcd(g,abs(val[node]-new_val));
      valid=0;
      return;
    }
  }
}
void solve(){ 
  cin>>n>>m;
  for(ll i=1;i<=m;i++){
    ll x,y; cin>>x>>y;
    adj[x].push_back({y,1});
    adj[y].push_back({x,-1});
  }
  while(1){
    valid=1;
    for(ll i=1;i<=n;i++){
      val[i]=-1;
    }
    for(ll i=1;i<=n;i++){
      if(val[i]==-1){
        val[i]=0;
        dfs(i);
      }
    }
    if(valid){
      break;
    }
  }
  vector<ll> ans;
  for(ll i=1;i<=n;i++){
    if((g%i)==0){
      ans.push_back(i);
    }
  }
  cout<<sz(ans)<<nline;
  for(auto i:ans){
    cout<<i<<" ";
  }
  cout<<nline;
  return;    
}  
int main()                                                                                 
{         
  ios_base::sync_with_stdio(false);                         
  cin.tie(NULL);                              
  ll test_cases=1;                 
  //cin>>test_cases;
  while(test_cases--){
    solve();
  }
  cout<<fixed<<setprecision(12);  
  cerr<<"Time:"<<1000*((double)clock())/(double)CLOCKS_PER_SEC<<"ms\n"; 
}