#include <bits/stdc++.h>
 
#define _overload(_1,_2,_3,name,...) name
#define _rep(i,n) _range(i,0,n)
#define _range(i,a,b) for(int i=(int)(a);i<(int)(b);++i)
#define rep(...) _overload(__VA_ARGS__,_range,_rep,)(__VA_ARGS__)
 
#define _rrep(i,n) _rrange(i,n,0)
#define _rrange(i,a,b) for(int i=(int)(a)-1;i>=(int)(b);--i)
#define rrep(...) _overload(__VA_ARGS__,_rrange,_rrep,)(__VA_ARGS__)
 
#define _all(arg) begin(arg),end(arg)
#define uniq(arg) sort(_all(arg)),(arg).erase(unique(_all(arg)),end(arg))
#define bit(n) (1LL<<(n))
 
// #define DEBUG
 
#ifdef DEBUG
    #define dump(...) fprintf(stderr, __VA_ARGS__)
#else
    #define dump(...)
#endif
 
template<class T>bool chmax(T &a, const T &b) { return (a<b)?(a=b,1):0;}
template<class T>bool chmin(T &a, const T &b) { return (b<a)?(a=b,1):0;}
 
using namespace std;
using ll=long long;
using vi=vector<int>;
using vll=vector<ll>;
 
const double EPS = 1e-10;
const double PI = acos(-1.0);
const int inf = INT_MAX;
const ll INF = LLONG_MAX;
const ll MOD = 1000000007LL;
const int dx[4]={1,0,-1,0};
const int dy[4]={0,1,0,-1};
 
ll extgcd(ll a,ll b,ll& x,ll& y){x=1,y=0;ll g=a;if(b!=0) g=extgcd(b,a%b,y,x),y-=a/b*x;return g;}
ll ADD(const ll &a, const ll &b, const ll mod=MOD) { return (a+b)%mod;}
ll SUB(const ll &a, const ll &b, const ll mod=MOD) { return (a-b+mod)%mod;}
ll MUL(const ll &a, const ll &b, const ll mod=MOD) { return (1LL*a*b)%mod;}
ll DIV(const ll &a, const ll &b, const ll mod=MOD) {ll x,y; extgcd(b,mod,x,y);return MUL(a,(x+mod)%mod,mod);}
 
vector<vi> G;

vi C, used;

bool dfs(int v, int root){
    if(v == root){
        return true;
    }
    if(root < 0) root = v;

    if(used[v]) return false;
    used[v] = true;

    bool ret = false;
    for(auto& nv : G[v]){
        if(ret |= dfs(nv, root)){
            C.emplace_back(v);
            break;
        }
    }
    return ret;
}

int main(void){
    cin.tie(0);
    ios::sync_with_stdio(false);

    int n, m; cin >> n >> m;

    G = vector<vi>(n);
    rep(_, m){
        int a, b; cin >> a >> b;
        a--, b--;
        G[a].emplace_back(b);
    }

    rep(v, n){
        used = vi(n);
        if(dfs(v, -1)) break;
    }
    if(C.size() == 0){
        cout << -1 << endl;
        return 0;
    }
    reverse(_all(C));

    int K;
    while(1){
        K = C.size();
        set<int> s(_all(C));
        vector<vi> T(n);
        int k = -1;
        rep(i, K){
            int v = C[i];
            for(auto& e : G[v]){
                if(s.find(e) != end(s)){
                    T[v].emplace_back(e);
                }
            }
            if(T[v].size() != 1){
                k = i;
                break;
            }
        }
        if(k < 0){
            break;
        }

        int k_s = -1;
        [&](){
            for(auto& e : G[C[k]]){
                if(e != C[(k + 1) % K]){
                    rep(i, K){
                        if(C[i] == e){
                            k_s = i;
                            return;
                        }
                    }
                }
            }
        }();

        vi D;
        while(1){
            D.emplace_back(C[k_s]);
            if(k_s == k) break;
            (++k_s) %= K;
        }

        C = D;
    }

    cout << K << endl;
    for(auto& v : C){
        cout << v + 1 << endl;
    }

    return 0;
}


Measured memory_bytes: 512