#include <bits/stdc++.h>
using namespace std;
#define repr(i,a,b) for(int i=a;i<b;i++)
#define rep(i,n) for(int i=0;i<n;i++)
#define reprrev(i,a,b) for(int i=b-1;i>=a;i--) // [a, b)
#define reprev(i,n) reprrev(i,0,n)
typedef long long ll;
typedef unsigned long long ull;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }

const ll mod = 1e9+7;


void chmod(ll &M){
    if(M >= mod) M %= mod;
    else if(M < 0){
        M += (abs(M)/mod + 1)*mod;
        M %= mod;
    }
}

ll modpow(ll x, ll n){
    if(n==0) return 1;
    ll res=modpow(x, n/2);

    if(n%2==0) return res*res%mod;
    else return res*res%mod*x%mod;
}

int getl(int i, int N) { return i==0? N-1:i-1; };
int getr(int i, int N) { return i==N-1? 0:i+1; };

// 線分 ab の偏角 返り値は[-π, π]
double argument(const pair<double, double> &a, const pair<double, double> &b){
    double ax=a.first, ay=a.second, bx=b.first, by=b.second;
    return atan2(by-ay, bx-ax);
}

/* <-----------------------------------------------------------------------------------> */
/* <-----------------------------------------------------------------------------------> */
/* <-----------------------------------------------------------------------------------> */
/* <-----------------------------------------------------------------------------------> */
/* <-----------------------------------------------------------------------------------> */
/* <-----------------------------------------------------------------------------------> */
/* <-----------------------------------------------------------------------------------> */
/* <-----------------------------------------------------------------------------------> */


/* 注意 

    1LL<<60 * N とかのオーバーフローに気を付けろ
    制約がヒントになる、ちゃんと全ての制約を見ろ

*/


// 有向グラフでの閉路検出
// O(V+E)
struct AcyclicCheck {
    int NMAX;
    vector<int> status;
    // 0:New, 1:Active, 2:Finished
    // Activeは行きがけ順序、Finishedは帰り掛け順序
    vector<vector<int>> const * g;

    AcyclicCheck(int NMAX, vector<vector<int>> const & G) :NMAX(NMAX) {
        status.resize(NMAX, 0);
        g = &G;
    }
    
    bool isAcyclicDFS(int v) {
        status[v] = 1;
        for(auto to : (*g)[v]) {
            if(status[to] == 1) return false;
            else if(status[to] == 0) {
                if(!isAcyclicDFS(to)) return false;
            }
        }
        status[v] = 2;
        return true;
    }

    bool isAcyclic() {
        rep(i, NMAX) status[i] = 0;
        rep(i, NMAX) if(status[i] == 0) {
            if(!isAcyclicDFS(i)) return false;
        }
        return true;
    }
};

int N, M;
vector<vector<int>> adj;

void input() {
    cin >> N >> M;
    adj.resize(N);
    rep(i, M) {
        int a, b; cin >> a >> b;
        --a; --b;
        adj[a].push_back(b);
    }
}

vector<int> bfs(int s) {
    int const inf = 1<<29;
    vector<int> dist(N, inf), pre(N, -1);
    queue<int> qu;
    qu.push(s);
    dist[s] = 0;

    while(!qu.empty()) {
        int now = qu.front(); qu.pop();
        for(auto to: adj[now]) {
            if(dist[to] > dist[now]+1) {
                dist[to] = dist[now] + 1;
                pre[to] = now;
                qu.push(to);
            }
        }
    }

    pair<int, int> best(inf, -1);
    rep(i, N) if (i != s && dist[i] != inf) {
        for(auto j: adj[i]) if (j == s) {
            chmin(best, make_pair(dist[i], i));
        }
    }

    if (best.first == inf) { return vector<int>(N+1, 0); }
    else {
        vector<int> res;
        int now = best.second;
        while(now != -1) {
            res.push_back(now);
            now = pre[now];
        }
        return res;
    }
}

void solve() {
    AcyclicCheck ac(N, adj);
    if (ac.isAcyclic()) { cout << -1 << endl; return; }

    vector<int> ans(N+1, 0);
    rep(i, N) {
        auto tmp = bfs(i);
        if(ans.size() > tmp.size()) ans = tmp;
    }

    cout << ans.size() << endl;
    for(auto v: ans) cout << v+1 << endl;
}

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

    input();
    solve();
    
    return 0;
}


Measured memory_bytes: 384