#include <iostream>
#include <vector>
#include <map>
#include <algorithm>


using namespace std;

using ll = long long;

int cycle_start = -1, cycle_end;

bool dfs(int v, const vector<vector<int>> &a, vector<int> &used, map<int, int>& p) {
    used[v] = 1;
    for (auto to : a[v]) {
        if (used[to] == 0) {
            p[to] = v;
            if (dfs(to, a, used, p)) {
                return true;
            }
        } else if (used[to] == 1) {
            cycle_start = v;
            cycle_end = to;
            return true;
        }
    }
    used[v] = 2;
    return false;
}

int main() {
    int n, m;
    cin >> n >> m;
    int u, v;
    vector<vector<int>> a(n);
    for (int to = 0; to < m; ++to) {
        cin >> v >> u;
        a[u - 1].push_back(v - 1);
    }
    vector<int> used(n);
    map<int, int> p;
    vector<int> f;
    for (int to = 0; to < n; ++to) {
            if(dfs(to, a, used, p)) {
                break;
            }

    }

    if  (cycle_start == -1) {
        std::cout << -1;
        return 0;
    }
    vector<int> cycle;
    cycle.push_back (cycle_start);
    for (int v=cycle_end; v!=cycle_start && v.co; v=p[v]) {
        cycle.push_back (v);
        std::cout << v << "\n";
    }
    cycle.push_back (cycle_start);
    reverse (cycle.begin(), cycle.end());
    std::cout << cycle.size() << "\n";
    for (auto v: cycle) {
        std::cout << v << " ";
    }
    return 0;
}