#pragma GCC optimize("Ofast")
#include<bits/stdc++.h>
using namespace::std;
struct __INIT{__INIT(){cin.tie(0);ios::sync_with_stdio(false);cout<<fixed<<setprecision(15);}} __init;

// #include <boost/multiprecision/cpp_dec_float.hpp>
// #include <boost/multiprecision/cpp_int.hpp>
// #include <boost/numeric/interval.hpp>
// #include <boost/numeric/interval/io.hpp>
// #include <boost/intrusive/rbtree.hpp>
// #include <boost/geometry.hpp>
// #include <boost/geometry/geometries/linestring.hpp>
// #include <boost/geometry/geometries/polygon.hpp>
// #include <boost/geometry/geometries/point_xy.hpp>
// namespace mp = boost::multiprecision;
// typedef mp::number<mp::cpp_dec_float<0>> cfloat;
// typedef mp::cpp_int cint;

#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>
#include<ext/pb_ds/tag_and_trait.hpp>
using namespace __gnu_pbds;
template<typename T=long long>using pbds=tree<T,null_type,less<T>,rb_tree_tag,tree_order_statistics_node_update>;

typedef long long lint;
typedef long long ll;
typedef long double ldouble;
typedef vector<lint> vec;
typedef vector<vector<lint>> mat;
typedef vector<vector<vector<lint>>> mat3;
typedef vector<double> dvec;
typedef vector<vector<double>> dmat;
typedef vector<vector<vector<double>>> dmat3;
typedef vector<string> svec;
typedef vector<vector<string>> smat;
typedef vector<vector<vector<string>>> smat3;
typedef vector<pair<lint,lint>> pvec;
typedef vector<vector<pair<lint,lint>>> pmat;
typedef vector<vector<vector<pair<lint,lint>>>> pmat3;
#define rep(i, n) for(lint i = 0; i < (lint)(n); i++)
#define irep(i) for(lint i = 0;; i++)
#define irep1(i) for(lint i = 1;; i++)
#define irep2(i) for(lint i = 2;; i++)
#define rrep(i, n) for(lint i = (lint)(n-1); i >-1; i--)
#define rrepi(i,a,b) for(lint i = (lint)(b-1); i >a-1; i--)
#define repi(i,a,b) for(lint i=lint(a);i<lint(b);i++)
#define rep2(i,a,b,c) for(lint i=lint(a);i>lint(b);i+=c)
#define all(x) (x).begin(),(x).end()
#define PI 3.141592653589793
#define dist(x1,y1,x2,y2) (pow(pow(x2-x1,2)+pow(y2-y1,2),0.5))
#define output(v) do{bool f=0;for(auto i:v){cout<<(f?" ":"");if(i>INF/2)cout<<"INF";else cout<<i;f=1;}cout<<"\n";}while(0)
#define output2(v) for(auto j:v)output(j);
#define _output(v) do{bool f=0;rep(i,v.size()){cout<<(f?" ":"");if(v[i]>INF/2)cout<<"INF";else cout<<v[i];f=1;}cout<<"\n";}while(0)
#define _output2(v) rep(j,v.size()){_output(v[j]);}
#define input(a,n) lint n;cin>>n;vector<lint>a(n);rep(i,n)cin>>a[i];
#define SUM(v) accumulate(all(v),0LL)
#define INF (1LL<<60)
#define IINF (1<<30)
#define EPS (1e-10)
#define LINF 9223372036854775807
#define MOD 1000000007
#define endl "\n"
template<typename T=lint>T in(){return *istream_iterator<T>(cin);}
inline lint gcd(lint a,lint b){return b?gcd(b,a%b):a;}
inline lint lcm(lint a,lint b){return a*b/gcd(a,b);}
inline bool chmin(auto& s,const auto& t){bool res=s>t;s=min(s,t);return res;}
inline bool chmax(auto& s,const auto& t){bool res=s<t;s=max(s,t);return res;}
vector<lint> dx={-1,1,0,0,1,1,-1,-1};
vector<lint> dy={0,0,-1,1,1,-1,1,-1};



int main(){
    lint n,m;
    cin>>n>>m;
    vec a(m),b(m);
    mat l(n);
    rep(i,m){
        cin>>a[i]>>b[i];
        a[i]--;b[i]--;
        l[a[i]].push_back(b[i]);
    }
    vec diff(n,INF);
    diff[0]=0;
    rep(i,n-1)rep(i,m){
        chmin(diff[b[i]],diff[a[i]]-1);
    }
    lint st=-1;
    rep(i,m){
        if(chmin(diff[b[i]],diff[a[i]]-1)){
            st=a[i];
            break;
        }
    }
    if(st==-1){
        cout<<-1<<endl;
        return 0;
    }
    queue<pair<lint,vector<lint>>>que;
    que.emplace(st,vector<lint>{});
    lint cnt=-1;
    vec ans2;
    while(!que.empty()){
        lint now;
        vec ans;
        tie(now,ans)=que.front();
        if(cnt!=-1&&cnt<(lint)ans.size())break;
        que.pop();
        if(find(all(ans),now)!=ans.end()&&ans.size()!=0){
            bool f=0;
            cnt=ans.size();
            rep(i,ans.size()){
                if(ans[i]==now){
                    if(ans2.size()==0||ans2.size()>ans.size()-i){
                        ans2.clear();
                        f=1;
                    }
                    else break;
                }
                if(f)ans2.push_back(ans[i]+1);
            }
        }
        ans.push_back(now);
        for(lint to:l[now]){
            que.emplace(to,ans);
        }
    }
    cout<<ans2.size()<<endl;
    rep(i,ans2.size()){
        cout<<ans2[i]<<endl;
    }
}

Measured memory_bytes: 4096