#include <bits/stdc++.h>
#include <boost/multiprecision/cpp_int.hpp>
using namespace std;
//#define int long long
using bll = boost::multiprecision::cpp_int;
using ll = long long;
//constexpr int INF = 1e9;//INT_MAX=(1<<31)-1=2147483647
constexpr ll INF = (ll)1e18;//(1LL<<63)-1=9223372036854775807
constexpr ll MOD = (ll)1e9 + 7;
constexpr double EPS = 1e-9;
constexpr int dx[4]={1,0,-1,0};
constexpr int dy[4]={0,1,0,-1};
#define p(var) std::cout<<var<<std::endl
#define PI (acos(-1))
#define rep(i, n)   for(ll i=0, i##_length=(n); i< i##_length; ++i)
#define repeq(i, n) for(ll i=1, i##_length=(n); i<=i##_length; ++i)
#define all(v) (v).begin(), (v).end()
#define uniq(v) (v).erase(unique((v).begin(), (v).end()), (v).end());
template<typename T> inline void pv(vector<T> v) { for(ll i=0, N=v.size(); i<N; i++) cout<< v[i] << (i==N-1 ? '\n' : ' '); }
template<typename T> inline T gcd(T a, T b) { return b ? gcd(b,a%b) : a; }
template<typename T> inline T lcm(T a, T b) { return a / gcd(a,  b) * b; }
template<typename T1, typename T2> inline T1 power(T1 x, T2 n){ return n ? power(x*x%MOD,n/2)*(n%2?x:1)%MOD : 1; }
template<typename T1, typename T2> inline bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }
template<typename T1, typename T2> inline bool chmin(T1 &a, T2 b) { return a > b && (a = b, true); }
template<typename T> class dvector : public std::vector<T> {
public:
	dvector() : std::vector<T>() {}
	explicit dvector(size_t n, const T& value = T()) : std::vector<T>(n,value) {}
	dvector(const std::vector<T>& v) : std::vector<T>(v) {}
	dvector(const std::initializer_list<T> il) : std::vector<T>(il) {}
	dvector(const typename std::vector<T>::iterator first, const typename std::vector<T>::iterator last) : std::vector<T>(first, last) {}
	dvector(const std::string::iterator first, const std::string::iterator last) : std::vector<T>(first, last) {}
	T& operator[](size_t n) { return this->at(n); }
	const T& operator[](size_t n) const { return this->at(n); }
};
class dbool{
public:
	dbool() : boolvalue(false) {}
	dbool(bool b) : boolvalue(b) {}
	dbool(const dbool &b) : boolvalue(b.boolvalue) {}
	operator bool&() { return boolvalue; }
	operator const bool&() const { return boolvalue; }
private:
	bool boolvalue;
};
template<typename T1, typename T2> ostream& operator<<(ostream& s, pair<T1, T2>& p) {return s << "(" << p.first << ", " << p.second << ")";}
template<typename T> ostream& operator<<(ostream& s, dvector<T>& v) {
	for (int i = 0, len = v.size(); i < len; ++i){ s << v[i]; if (i < len - 1) s << "\t"; } return s; }
template<typename T> ostream& operator<<(ostream& s, dvector< dvector<T> >& vv) {
	for (int i = 0, len = vv.size(); i < len; ++i){ s << vv[i] << endl; } return s; }
template<typename T1, typename T2> ostream& operator<<(ostream& s, map<T1, T2>& m) {
	s << "{" << endl; for (auto itr = m.begin(); itr != m.end(); ++itr){ s << "\t" << (*itr).first << " : " << (*itr).second << endl; } s << "}" << endl; return s; }
template<typename T> ostream& operator<<(ostream& s, set<T>& se) {
	s << "{ "; for (auto itr = se.begin(); itr != se.end(); ++itr){ s << (*itr) << "\t"; } s << "}" << endl; return s; }
template<typename T> ostream& operator<<(ostream& s, multiset<T>& se) {
	s << "{ "; for (auto itr = se.begin(); itr != se.end(); ++itr){ s << (*itr) << "\t"; } s << "}" << endl; return s; }
#ifdef LOCAL_DEV
	#define debug(var) std::cout<<#var" = "<<var<<std::endl
#else
	#define debug(var)
#endif
#ifdef LOCAL_TEST
	#define vector dvector
	#define bool dbool
#endif

/*-----8<-----8<-----*/

//gに閉路を検出したときtrueを返す
//seen[v] := 行きがけ順の意味で v が訪問済みであることを表す 
//　　　　　　(帰りがけ順の意味で v が訪問済みかどうかは定かではない)
//finished[v] := 帰りがけ順の意味で v が訪問済みであることを表す
//history := 探索した頂点の履歴
ll dfscycle(vector<vector<ll>> &g, ll v, ll p, vector<bool> &seen, vector<bool> &completed, vector<ll> &history){
	history.push_back(v);
	seen[v]=true;

	for(ll nextv:g[v]){
		//if(nextv==p)continue;//無向グラフの場合はコメントアウト外すこと
		if(completed[nextv])continue;
		if(seen[nextv] && !completed[nextv])return nextv;
		ll returnedv=dfscycle(g,nextv,v,seen,completed,history);
		if(returnedv!=-1)return returnedv;
	}

	history.pop_back();
	completed[v]=true;
	return -1;
}

//閉路を構成する頂点を返す、閉路がない場合は空のvectorが返される
vector<ll> searchcycle(vector<vector<ll>> &g, ll v){
	vector<bool> seen(g.size(),false);
	vector<bool> completed(g.size(),false);
	vector<ll> history;
	ll returnedv=dfscycle(g,v,-1,seen,completed,history);
	if(returnedv==-1)return vector<ll>();

	vector<ll> cyclev;
	while(!history.empty()){
		ll v=history.back();
		cyclev.push_back(v);
		history.pop_back();
		if(v==returnedv)break;
	}
	reverse(cyclev.begin(), cyclev.end());
	return cyclev;
}


//弦を持たない最小な閉路を構成する頂点を返す
vector<ll> minimizecycle(vector<vector<ll>> &g, vector<ll> cyclev){
	ll gsize=g.size();
	vector<ll> mincyclev=cyclev;
	while(1){
		ll csize=mincyclev.size();
		ll fromindex=-1,toindex=-1;
		vector<ll> order(gsize,-1);
		for(ll i=0;i<csize;i++)order[mincyclev[i]]=i;
		for(ll i=0;i<csize;i++){
			for(ll nextv:g[mincyclev[i]]){
				//nextvが閉路を構成する頂点 && nextvは想定している閉路の次の頂点でない
				if(order[nextv]!=-1 && mincyclev[(i+1)%csize]!=nextv){
					fromindex=i;
					toindex=order[nextv];
				}
			}
		}

		if(fromindex==-1)break;

		vector<ll> newcyclev;
		newcyclev.push_back(mincyclev[fromindex]);
		ll index=toindex;
		while(index!=fromindex){
			newcyclev.push_back(mincyclev[index]);
			index=(index+1)%csize;
		}
		mincyclev=newcyclev;
	}
	return mincyclev;
}

signed main() {
	//ABC142F
	//https://atcoder.jp/contests/abc142/tasks/abc142_f
	ll N,M;
	cin>>N>>M;

	vector<vector<ll>> g(N);
	rep(i,M){
		ll a,b;
		cin>>a>>b;
		a--;b--;
		g[a].push_back(b);
	}

	vector<ll> cyclev;
	if([&]{
		rep(i,N){
			cyclev = searchcycle(g, i);
			if(cyclev.size()>0)return false;
		}
		return true;
	}()){
		p(-1);
		return 0;
	}

	vector<ll> mincyclev = minimizecycle(g, cyclev);
	
	p(mincyclev.size());
	rep(i,mincyclev.size())p(1+mincyclev[i]);
	return 0;
}


Measured memory_bytes: 384