#include <bits/stdc++.h>
using namespace std;
#define INF 1<<30
#define endl '\n'
#define maxn 100005
#define tc printf("Case %d: ", cs)
#define tcn printf("Case %d:\n", cs);
#define FASTIO ios_base::sync_with_stdio(false), cin.tie(0), cout.tie(0);
typedef long long ll;
const double PI = acos(-1.0);

#define dbg1(x) cerr << #x << " = " << x << endl;
#define dbg2(x, y) cerr << #x << " = " << x << ", " << #y << " = " << y << endl;
#define dbg3(x, y, z) cerr << #x << " = " << x << ", " << #y << " = " << y << ", " << #z << " = " << z << endl;
#define dbg4(w,x, y, z) cerr << #w << " = " << w << ", " <<#x << " = " << x << ", " << #y << " = " << y << ", " << #z << " = " << z << endl;

template < typename F, typename S >
ostream& operator << ( ostream& os, const pair< F, S > & p ) {
    return os << "(" << p.first << ", " << p.second << ")";
}

template < typename T >
ostream &operator << ( ostream & os, const vector< T > &v ) {
    os << "{";
    for (auto it = v.begin(); it != v.end(); ++it) {
        if ( it != v.begin() ) os << ", ";
        os << *it;
    }
    return os << "}";
}

template < typename T >
ostream &operator << ( ostream & os, const set< T > &v ) {
    os << "[";
    for (auto it = v.begin(); it != v.end(); ++it) {
        if ( it != v.begin()) os << ", ";
        os << *it;
    }
    return os << "]";
}

template < typename F, typename S >
ostream &operator << ( ostream & os, const map< F, S > &v ) {
    os << "[";
    for (auto it = v.begin(); it != v.end(); ++it) {
        if ( it != v.begin() ) os << ", ";
        os << it -> first << " = " << it -> second ;
    }
    return os << "]";
}

#define dbg(args...) do {cerr << #args << " : "; faltu(args); } while(0)

clock_t tStart = clock();
#define timeStamp dbg("Execution Time: ", (double)(clock() - tStart)/CLOCKS_PER_SEC)

void faltu () { cerr << endl; }

template <typename T>
void faltu( T a[], int n ) {
    for (int i = 0; i < n; ++i) cerr << a[i] << ' ';
    cerr << endl;
}

template <typename T, typename ... hello>
void faltu( T arg, const hello &... rest) { cerr << arg << ' '; faltu(rest...); }
// Program showing a policy-based data structure.
#include <ext/pb_ds/assoc_container.hpp> // Common file 
#include <ext/pb_ds/tree_policy.hpp>
#include <functional> // for less 
#include <iostream>
using namespace __gnu_pbds;
using namespace std;
// GNU link : https://goo.gl/WVDL6g
typedef tree<int, null_type, less_equal<int>, rb_tree_tag,
        tree_order_statistics_node_update>
        new_data_set;

/**___________________________________________________**/

const ll Mod = 1e9 + 7;
ll n, m;
vector<vector<ll>> graph, back;
vector<ll> dp;

void bfs(ll u)
{
    for (auto &I : dp) {I = Mod;}

    queue<ll>Q;
    for (auto &x : graph[u]) {
        dp[x] = 1;
        Q.push(x);
    }

    while (!Q.empty()) {
        ll v = Q.front();
        Q.pop();
        for (auto &to : graph[v]) {
            if (dp[to] > dp[v] + 1) {
                dp[to] = dp[v] + 1;
                Q.push(to);
            }
        }
    }
}

int main()
{
    FASTIO
    /*
#ifndef ONLINE_JUDGE
    freopen("in.txt", "r", stdin);
    freopen("out.txt", "w", stdout);
    freopen("error.txt", "w", stderr);
#endif
//*/
    cin >> n >> m;
    graph.resize(n);
    back.resize(n);
    dp.resize(n);
    for (int i = 0; i < m; i++) {
        ll u, v;
        cin >> u >> v;
        --u; --v;
        graph[u].push_back(v);
        back[v].push_back(u);
    }

    ll mn = Mod, idx = -1;
    for (int i = 0; i < n; i++) {
        bfs(i);
        if (dp[i] < mn) {
            mn = dp[i];
            idx = i;
        }
    }
    if (idx == -1) {
        cout << -1 << endl;
        return 0;
    }
    bfs(idx);
    cout << dp[idx] << endl;
    int x = idx;
    while (dp[x] > 1) {
        cout << x + 1 << endl;
        for (auto &v : back[x]) {
            if (dp[v] + 1 == dp[x]) {
                x = v;
                break;
            }
        }
    }
    cout << x + 1 << endl;
}

Measured memory_bytes: 384