#include<bits/stdc++.h>
using namespace std;

#define endl '\n'
#define ALL(a)  (a).begin(),(a).end()
#define ALLR(a)  (a).rbegin(),(a).rend()
#define spa << " " <<
#define lfs <<fixed<<setprecision(10)<<
#define test cout<<"test"<<endl;
#define fi first
#define se second
#define MP make_pair
#define PB push_back
#define EB emplace_back
#define rep(i,n,m) for(ll i = n; i < (ll)(m); i++)
#define rrep(i,n,m) for(ll i = n - 1; i >= (ll)(m); i--)
using ll = long long;
using ld = long double;
const ll MOD = 1e9+7;
//const ll MOD = 998244353;
const ll INF = 1e18;
using P = pair<ll, ll>;
template<typename T>
void chmin(T &a,T b){if(a>b)a=b;}
template<typename T>
void chmax(T &a,T b){if(a<b)a=b;}
void pmod(ll &a,ll b){a=(a+b)%MOD;}
void pmod(ll &a,ll b,ll c){a=(b+c)%MOD;}
void qmod(ll &a,ll b){a=(a*b)%MOD;}
void qmod(ll &a,ll b,ll c){a=(b*c)%MOD;}
ll median(ll a,ll b, ll c){return a+b+c-max({a,b,c})-min({a,b,c});}
void ans1(bool x){if(x) cout<<"Yes"<<endl;else cout<<"No"<<endl;}
void ans2(bool x){if(x) cout<<"YES"<<endl;else cout<<"NO"<<endl;}
void ans3(bool x){if(x) cout<<"Yay!"<<endl;else cout<<":("<<endl;}
template<typename T1,typename T2>
void ans(bool x,T1 y,T2 z){if(x)cout<<y<<endl;else cout<<z<<endl;}  
template<typename T>
void debug(vector<vector<T>>v,ll h,ll w){for(ll i=0;i<h;i++)
{cout<<v[i][0];for(ll j=1;j<w;j++)cout spa v[i][j];cout<<endl;}};
void debug(vector<string>v,ll h,ll w){for(ll i=0;i<h;i++)
{for(ll j=0;j<w;j++)cout<<v[i][j];cout<<endl;}};
template<typename T>
void debug(vector<T>v,ll n){if(n!=0)cout<<v[0];
for(ll i=1;i<n;i++)cout spa v[i];cout<<endl;};
template<typename T>
vector<vector<T>>vec(ll x, ll y, T w){
  vector<vector<T>>v(x,vector<T>(y,w));return v;}
ll gcd(ll x,ll y){ll r;while(y!=0&&(r=x%y)!=0){x=y;y=r;}return y==0?x:y;}
template<typename T>
vector<ll>dx={1,0,-1,0,1,1,-1,-1};
vector<ll>dy={0,1,0,-1,1,-1,1,-1};
template<typename T>
vector<T> make_v(size_t a,T b){return vector<T>(a,b);}
template<typename... Ts>
auto make_v(size_t a,Ts... ts){
  return vector<decltype(make_v(ts...))>(a,make_v(ts...));
}

ll MAX_V = 1100;
vector<vector<ll>> G(MAX_V);//隣接リスト(添え字は始点、値は終点)
vector<ll>t(MAX_V);//訪問したかどうか
bool sw=false;
stack<ll>st;
ll star;
void dfs(ll k){
  t[k]=1;
  st.push(k);
  for(ll i=0;i<G[k].size();i++){
    if(sw)return;
    if(t[G[k][i]]==0){
      dfs(G[k][i]); 
    }
    else if(G[k][i]==star){
      st.push(G[k][i]);
      sw=true;
    }
  }
  if(sw)return;
  st.pop();
}


int main(){
  cin.tie(NULL);
  ios_base::sync_with_stdio(false);
  ll res=0,res1=INF,res2=-INF,buf=0;
  bool judge = true;
  ll n,m;cin>>n>>m;
  vector<ll>u(m),v(m);
  for(ll i=0;i<m;i++){
    ll p,q;
    cin>>p>>q;
    u[i]=p-1,v[i]=q-1;
    G[p-1].push_back(q-1);
  }
  rep(i,0,n){
    if(sw)break;
    fill(ALL(t),0),
    star=i;
    dfs(i);
  }
  if(sw){
    //cout<<sw<<endl;
    ll tmp=st.top();
    vector<ll>a;
    a.PB(tmp);
    st.pop();
    while(st.top()!=tmp){
      a.PB(st.top());
      st.pop();
    }
    map<ll,ll>mp;
    reverse(ALL(a));
    rep(i,0,a.size())mp[a[i]]=i;
    vector<ll>b(a.size());
    //debug(a,a.size());
    rep(i,0,m){
      if(mp.find(u[i])!=mp.end()&&mp.find(v[i])!=mp.end()){
        //cout<<i<<endl;
        //debug(b,a.size());
        ll x=mp[u[i]],y=mp[v[i]];
        //cout<<x spa y<<endl;
        if(x<y){
          rep(j,x+1,y)b[j]=-1;
        }
        else{
          rep(j,0,y-1)b[j]=-1;
          rep(j,x+1,a.size())b[j]=-1;
        }
      }
    }
    ll cnt=0;
    rep(i,0,a.size()){
      if(b[i]!=-1)cnt++;
    }
    cout<<cnt<<endl;
    rep(i,0,a.size()){
      if(b[i]!=-1)cout<<a[i]+1<<endl;
    }
  }
  else cout<<-1<<endl;
  
  return 0;
}

Measured memory_bytes: 512