#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for (int i = 0; i < int(n); i++)
#define rep1(i,n) for (int i = 1; i <= int(n); i++)
#define repR(i,n) for (int i = int(n)-1; i >= 0; i--)
#define rep1R(i,n) for (int i = int(n); i >= 1; i--)
#define all(x) (x).begin(), (x).end()
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define fst first
#define snd second
#define cauto const auto
#ifdef LOCAL
#define dump(...) cerr << "[" << __LINE__ << ":" << __FUNCTION__ << "] ", _dmp_(#__VA_ARGS__, __VA_ARGS__);
void _dmp_(const char*) { cerr << endl; }
template <class T, class... U>
void _dmp_(const char *s, T const& x, U const& ...y) { const char *o = "({[", *e = "]})"; for (int i = 0; *s != '\0'; cerr << *s++) { if (count(o,o+3,*s)) i++; if (count(e,e+3,*s)) i--; if (!i && *s == ',') break; } cerr << " = " << x; if (*s == ',') cerr << ", ", s++; _dmp_(s, y...); }
#else
#define dump(...)
#endif
using ll = long long;
#define int ll
template <class T> using pque_max = priority_queue<T>;
template <class T> using pque_min = priority_queue<T, vector<T>, greater<T> >;
#define CONAINER_OUTPUT os << "{"; for (auto const& x : v) os << " " << x; return os << " }";
template <class T> ostream& operator<<(ostream& os, vector<T> const& v) { CONAINER_OUTPUT }
template <class T> ostream& operator<<(ostream& os, deque<T> const& v) { CONAINER_OUTPUT }
template <class T> ostream& operator<<(ostream& os, set<T> const& v) { CONAINER_OUTPUT }
template <class T, class U> ostream& operator<<(ostream& os, map<T,U> const& v) { CONAINER_OUTPUT }
#undef CONAINER_OUTPUT
template <class T> istream& operator>>(istream& is, vector<T>& v) { for (auto& x : v) is >> x; return is; }
template <class T, class S> ostream& operator<<(ostream& os, pair<T,S> const& p) { return os << "(" << p.first << ", " << p.second << ")"; }
template <class T, class S> istream& operator>>(istream& is, pair<T,S> const& p) { return is >> p.first >> p.second; }
struct _Init_ { _Init_() { cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(20); } } _init_;

template <class T> void quit(T const& x) { cout << x << endl; exit(0); }
template <class T> bool chmin(T& x, T const& y) { if (x > y) { x = y; return true; } return false; }
template <class T> bool chmax(T& x, T const& y) { if (x < y) { x = y; return true; } return false; }
template <class It> auto sumof(It b, It e) { return accumulate(b,e,typename iterator_traits<It>::value_type{}); }
#define YesNo(c) ((c) ? "Yes" : "No")
////////////////////////////////
int n,m;
vector<vector<int> > g;
vector<int> d,v,used;

void dfs(int len, int x, int s) {
    used[x] = 1; used[s] = 0;
    if (len == 0) {
        if (x != s) return;
        cout << v.size() << "\n";
        for (int val : v) cout << 1+val << "\n";
        exit(0);
    }
    v.push_back(x);
    for (int y : g[x]) {
        if (used[y]) continue;
        dfs(len-1,y,s);
    }
    v.pop_back();    
}

const int inf = 1e9;

int32_t main() {
    cin >> n >> m;
    g.resize(n); used.resize(n); v.reserve(n), d.resize(n);
    rep (i,m) {
        int a,b; cin >> a >> b; a--,b--;
        g[a].push_back(b);
    }
    pair<int,int> p; p.fst = inf, p.snd = -1;
    rep (s,n) {
        rep (i,n) d[i] = -inf;
        queue<int> q;
        q.push(s); d[s] = 0;
        while (!q.empty()) {
            int x = q.front(); q.pop();
            used[x] = 1;
            for (int y : g[x]) {
                if (y == s) chmin(p, {d[x]+1,s});
                if (d[y] != -inf) continue;
                d[y] = d[x]+1;
                q.push(y);
            }
        }
    }
    if (p.snd == -1) quit(-1);
    rep (i,n) used[i] = 0;
    dfs(p.fst, p.snd, p.snd);
}


Measured memory_bytes: 384