#include<bits/stdc++.h>
using namespace std;using ll=long long;using vi=vector<int>;using vvi=vector<vi>;using vl=vector<ll>;using vvl=vector<vl>;using P=pair<int,int>;using PL=pair<ll,ll>;using vp=vector<P>;using vpl=vector<PL>;
template<typename T>constexpr auto inf=numeric_limits<T>::max()/2;constexpr int INF=inf<int>,MOD=1000000007;constexpr ll LINF=inf<ll>;
#define _ol3(_1,_2,_3,name,...)name
#define _rep(i,n)_repi(i,0,n)
#define _repi(i,a,b)for(int i=a,i##_l=(b);i<i##_l;++i)
#define REP(...)_ol3(__VA_ARGS__,_repi,_rep,)(__VA_ARGS__)
#define REPR(i,n)for(int i=n-1;i>=0;--i)
#define REPA(i,v)REP(i,(v).size())
#define all(v)(v).begin(),(v).end()
#define rall(v)(v).rbegin(),(v).rend()
#define bit(n)(1ll<<(n))
#define F first
#define S second
#define endl '\n'
#define cho(n,a,b)cout<<((n)?a:b)<<endl
void YES(int n){cho(n,"YES","NO");}void Yes(int n){cho(n,"Yes","No");}void Poss(int n){cho(n,"Possible","Impossible");}
void _print(ostream&){}template<class T,class...U>void _print(ostream&s,const T&t,const U&...u){s<<t<<(sizeof...(u)?' ':'\n');_print(s,u...);}
template<class...T>void print(const T&...t){_print(cout,t...);}template<class...T>void dprint(const T&...t){_print(cerr,t...);}
#ifndef LOCAL
struct osd{template<class T>osd&operator<<(const T&t){return*this;}};osd cer_;
#define dprint(...)
#define cerr cer_
#endif
template<class T>bool chmax(T&a,const T&b){bool x=a<b;x?a=b:b;return x;}template<class T>bool chmin(T&a,const T&b){bool x=a>b;x?a=b:b;return x;}
template<class T,class U>ostream&operator<<(ostream&o,const pair<T,U>&p){return o<<p.F<<' '<<p.S;}
template<class T,class U>istream&operator>>(istream&i,pair<T,U>&p){return i>>p.F>>p.S;}
template<class T>class iterable{static false_type c(string v);template<class U>static auto c(U v)->decltype(all(v),true_type());static false_type c(...);public:const static bool value=decltype(c(declval<T>()))::value;};
template<class T,typename=enable_if_t<iterable<T>::value>>ostream&operator<<(ostream&o,const T&v){for(auto&&i:v)o<<i<<' ';return o;}
template<class T>istream&operator>>(istream&i,vector<T>&v){for(T&j:v)i>>j;return i;}
template<class T>vector<T>&operator<<(vector<T>&v,const T&t){v.push_back(t);return v;}
template<class T>set<T>&operator<<(set<T>&v,const T&t){v.insert(t);return v;}
template<class T>multiset<T>&operator<<(multiset<T>&v,const T&t){v.insert(t);return v;}

struct way {
    int num;
    int parent_idx;
    way(int n, int parent_idx)
        :num(n)
        ,parent_idx(parent_idx){}
};

int main(){
    cin.tie(0);ios::sync_with_stdio(0);
    int n,m;
    cin>>n>>m;
    vvi g(n);
    REP(i,m){
        int a,b;
        cin>>a>>b;
        g[a-1].push_back(b-1);
    }
    stack<int> ans;
    REP(i,n){
        bitset<2000> passed;
        passed[i]=1;
        vector<way> ws;
        ws.emplace_back(i, -1);
        priority_queue<P, vp, greater<P>> q;//cost, ws index
        q.push({0,0});
        while(q.size()){
            P f=q.top();
            q.pop();
            for(int saki:g[ws[f.S].num]){
                dprint(ws[f.S].num,"=>",saki);
                if(saki==i){
                    dprint("found");
                    stack<int> a;
                    const way* pt=&ws[f.S];
                    while(1){
                        if(pt->parent_idx==-1)
                            dprint(pt->num,"=>NULL");
                        else
                            dprint(pt->num,"=>",ws[pt->parent_idx].num);

                        a.push(pt->num);
                        if(pt->parent_idx==-1)break;
                        pt=&ws[pt->parent_idx];
                    }
                    if(ans.size()==0 || ans.size()>a.size()){
                        swap(ans,a);
                    }
                    continue;
                }
                if(passed[saki])continue;
                ws.emplace_back(saki, f.S);
                q.push({f.F+1, ws.size()-1});
                passed[saki]=1;
            }
        }
    }
    if(ans.size()==0)
        cout<<-1<<endl;
    else{
        cout<<ans.size()<<endl;
        while(ans.size()){
            cout<<ans.top()+1<<' ';
            ans.pop();
        }
        cout<<endl;
    }
    return 0;
}


Measured memory_bytes: 384