#include "bits/stdc++.h"

// Begin Header {{{
using namespace std;

#ifndef DEBUG
#define dump(...)
#endif

#define all(x) x.begin(), x.end()
#define rep(i, b, e) for (intmax_t i = (b), i##_limit = (e); i < i##_limit; ++i)
#define reps(i, b, e) for (intmax_t i = (b), i##_limit = (e); i <= i##_limit; ++i)
#define repr(i, b, e) for (intmax_t i = (b), i##_limit = (e); i >= i##_limit; --i)
#define var(Type, ...) Type __VA_ARGS__; input(__VA_ARGS__)

constexpr size_t    operator""_zu(unsigned long long value) { return value; };
constexpr intmax_t  operator""_jd(unsigned long long value) { return value; };
constexpr uintmax_t operator""_ju(unsigned long long value) { return value; };

constexpr int INF = 0x3f3f3f3f;
constexpr intmax_t LINF = 0x3f3f3f3f3f3f3f3f_jd;

template <class T, class Compare = less<>>
using MaxHeap = priority_queue<T, vector<T>, Compare>;
template <class T, class Compare = greater<>>
using MinHeap = priority_queue<T, vector<T>, Compare>;

inline void input() {}
template <class Head, class... Tail>
inline void input(Head&& head, Tail&&... tail) {
    cin >> head;
    input(forward<Tail>(tail)...);
}

template <class T>
inline istream& operator>>(istream &is, vector<T> &vec) {
    for (auto &e: vec) {
        is >> e;
    }
    return is;
}

inline void output() { cout << "\n"; }
template <class Head, class... Tail>
inline void output(Head&& head, Tail&&... tail) {
    cout << head;
    if (sizeof...(tail)) {
        cout << " ";
    }
    output(forward<Tail>(tail)...);
}

template <class T>
inline ostream& operator<<(ostream &os, const vector<T> &vec) {
    static constexpr const char *delim[] = {" ", ""};
    for (const auto &e: vec) {
        os << e << delim[&e == &vec.back()];
    }
    return os;
}

template <class T>
inline vector<T> makeVector(const T &initValue, size_t sz) {
    return vector<T>(sz, initValue);
}

template <class T, class... Args>
inline auto makeVector(const T &initValue, size_t sz, Args... args) {
    return vector<decltype(makeVector<T>(initValue, args...))>(sz, makeVector<T>(initValue, args...));
}

template <class Func>
class FixPoint : Func {
public:
    explicit constexpr FixPoint(Func&& f) noexcept : Func(forward<Func>(f)) {}

    template <class... Args>
    constexpr decltype(auto) operator()(Args&&... args) const {
        return Func::operator()(*this, std::forward<Args>(args)...);
    }
};

template <class Func>
static inline constexpr decltype(auto) makeFixPoint(Func&& f) noexcept {
    return FixPoint<Func>{forward<Func>(f)};
}

template <class Container>
struct reverse_t {
    Container &c;
    reverse_t(Container &c) : c(c) {}
    auto begin() { return c.rbegin(); }
    auto end() { return c.rend(); }
};

template <class Container>
auto reversed(Container &c) {
    return reverse_t<Container>(c);
}

template <class T>
inline bool chmax(T &a, const T &b) noexcept {
    return b > a && (a = b, true);
}

template <class T>
inline bool chmin(T &a, const T &b) noexcept {
    return b < a && (a = b, true);
}

template <class T>
inline T diff(const T &a, const T &b) noexcept {
    return a < b ? b - a : a - b;
}
// End Header }}}

// Edge {{{
template <class Weight>
struct Edge {
    size_t from, to;
    Weight weight;

    Edge() {}
    Edge(size_t from, size_t to, Weight weight = 1) :
        from(from), to(to), weight(weight)
    {}

    bool operator<(const Edge &rhs) const {
        return weight < rhs.weight;
    }

    bool operator>(const Edge &rhs) const {
        return weight > rhs.weight;
    }

    operator size_t() const {
        return to;
    }
};
// }}}

// Graph {{{
template <class Weight>
class Graph : public vector<vector<Edge<Weight>>> {
    using graph = vector<vector<Edge<Weight>>>;

public:
    Graph() {}
    Graph(const size_t V) : graph(V) {}

    void connect(size_t from, size_t to, Weight weight = 1) {
        (*this)[from].emplace_back(from, to, weight);
    }

    friend ostream& operator<<(ostream &strm, const Graph &G) {
        for (size_t v = 0; v < G.size(); ++v) {
            strm << '[' << setw(2) << v << ']';
            for (const auto &e: G[v]) {
                strm << ' ' << setw(2) << e.to;
            }
            strm << '\n';
        }
        return strm;
    }
};
// }}}

// dijkstra {{{
template <class Weight>
vector<Weight> dijkstra(const Graph<Weight> &G, const vector<size_t> &startNodes, vector<int> &pre) {
    using P = pair<Weight, size_t>;
    vector<Weight> dp(G.size(), numeric_limits<Weight>::max());
    priority_queue<P, vector<P>, greater<>> pq;
    pre.resize(G.size(), -1);

    for (const auto startNode: startNodes) {
        dp[startNode] = 0;
        pq.emplace(0, startNode);
    }

    while (!pq.empty()) {
        const Weight curCost = pq.top().first;
        const size_t curNode = pq.top().second;
        pq.pop();

        if (dp[curNode] < curCost) {
            continue;
        }

        for (const auto &e: G[curNode]) {
            const Weight newCost = dp[curNode] + e.weight;
            const size_t newNode = e.to;
            if (newCost < dp[newNode]) {
                dp[newNode] = newCost;
                pq.emplace(newCost, newNode);
                pre[e.to] = curNode;
            }
        }
    }

    return dp;
}
// }}}

vector<int> getPath(const vector<int> &pre, const size_t g) {
    vector<int> ret;
    for (int cur = g; cur != -1; cur = pre[cur]) {
        ret.emplace_back(cur);
    }
    reverse(ret.begin(), ret.end());
    return ret;
}

vector<int> minCycle;

template <class T>
T minimumWeightCycle(Graph<T> &G) {
    const size_t V = G.size();
    T ret = numeric_limits<T>::max();
    for (size_t v = 0; v < V; ++v) {
        for (size_t i = 0; i < G[v].size(); ++i) {
            const Edge<T> e = G[v][i];
            const size_t u = e.to;
            G[v].erase(G[v].begin() + i);
            vector<int> pre;
            const auto dist = dijkstra(G, {u}, pre);
            if (dist[v] == numeric_limits<T>::max()) continue;
            if (ret > dist[v] + e.weight) {
                ret = dist[v] + e.weight;
                minCycle = getPath(pre, v);
            }
            G[v].insert(G[v].begin() + i, e);
        }
    }
    return ret;
}

signed main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.setf(ios_base::fixed);
    cout.precision(10);
    var(size_t, N, M);
    Graph<size_t> G(N);
    rep(i, 0, M) {
        var(size_t, a, b);
        a--, b--;
        G.connect(a, b);
    }
    const intmax_t res = minimumWeightCycle(G);
    dump(res);
    if (minCycle.empty()) {
        output(-1);
    } else {
        output(minCycle.size());
        for (const auto e: minCycle) output(e + 1);
    }
    return 0;
}


Measured memory_bytes: 384