//#include <bits/stdc++.h>
#include "bits/stdc++.h"
using namespace std;
typedef long long ll;
//#include "boost/multiprecision/cpp_int.hpp"
//typedef boost::multiprecision::cpp_int LL;
typedef long double dd;
#define i_7 (ll)(1E9+7)
//#define i_7 998244353
#define i_5 i_7-2

ll mod(ll a){
    ll c=a%i_7;
    if(c>=0)return c;
    return c+i_7;
}
typedef pair<ll,ll> l_l;
typedef pair<dd,dd> d_d;
ll inf=(ll)1E16;
#define rep(i,l,r) for(ll i=l;i<=r;i++)
#define pb push_back
ll max(ll a,ll b){if(a<b)return b;else return a;}
ll min(ll a,ll b){if(a>b)return b;else return a;}
void Max(ll &pos,ll val){pos=max(pos,val);}//Max(dp[n],dp[n-1]);
void Min(ll &pos,ll val){pos=min(pos,val);}
void Add(ll &pos,ll val){pos=mod(pos+val);}
dd EPS=1E-9;
#define fastio ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);
#define fi first
#define se second
#define endl "\n"
#define SORT(v) sort(v.begin(),v.end())
#define ERASE(v) v.erase(unique(v.begin(),v.end()),v.end())
#define POSL(v,x) (lower_bound(v.begin(),v.end(),x)-v.begin())
#define POSU(v,x) (upper_bound(v.begin(),v.end(),x)-v.begin())
//template<class T>void max(T a,T b){if(a<b)return b;else return a;}
//template<class T>void min(T a,T b){if(a>b)return b;else return a;}
//template<class T>bool Max(T&a, T b){if(a < b){a = b;return 1;}return 0;}
//template<class T>bool Min(T&a, T b){if(a > b){a = b;return 1;}return 0;}

//////////////////////////

#define N 1005
ll n,m;
vector<ll>v[N];
vector<ll>sta;
ll used[N];
vector<ll>cycle;

void dfs(ll id,ll par){
    if(cycle.size()>=1)return;
    sta.pb(id);
    used[id]=1;
    for(auto x:v[id]){
        if(cycle.size()>=1)return;
        if(x==par)continue;
        if(used[x]==2)continue;
        if(used[x]==1){
            while(1){
                ll top=sta[sta.size()-1];
                sta.pop_back();
                cycle.pb(top);
                if(top==x)return;
            }
        }else{
            dfs(x,id);
        }
    }
    if(cycle.size()>=1)return;
    sta.pop_back();
    used[id]=2;
}

vector<ll>chan(vector<ll>a){
    set<ll>se;for(auto x:a)se.insert(x);
    for(auto x:a){
        vector<ll>z;
        for(auto y:v[x]){
            if(se.find(y)!=se.end())z.pb(y);
        }
        v[x]=z;
    }
    ll as=a.size();
    
    rep(i,0,as-1){
        if(v[a[i]].size()>=2){
            ll to=v[a[i]][0];
            if(to==a[(i+1)%as])to=v[a[i]][1];
            ll j=0;
            rep(k,0,as-1)if(a[k]==to){j=k;break;}
            vector<ll>res;
            if(i<j){
                rep(x,j,as-1)res.pb(a[x]);
                rep(x,0,i)res.pb(a[x]);
            }else{
                rep(x,j,i)res.pb(a[x]);
            }
            return res;
        }
    }
    return a;
}

int main(){fastio
    cin>>n>>m;
    while(m--){
        ll a,b;cin>>a>>b;
        v[a].pb(b);
    }
    rep(i,1,n){
        dfs(i,-1);
        if(cycle.size()>=1)break;
    }
    reverse(cycle.begin(),cycle.end());
    //for(auto x:cycle)cout<<x<<" ";cout<<endl;
    if(cycle.size()==0){cout<<-1<<endl;return 0;}
    while(1){
        ll cs=cycle.size();
        cycle=chan(cycle);
        ll ds=cycle.size();
        if(cs==ds){
            cout<<cycle.size()<<endl;
            for(auto x:cycle)cout<<x<<endl;
            return 0;
        }
    }
    
    return 0;
}


Measured memory_bytes: 512