#include <bits/stdc++.h>
using namespace std;

using i64 = int64_t;

#define rep(i, x, y) for (i64 i = i64(x), i##_max_for_repmacro = i64(y); i < i##_max_for_repmacro; ++i)
#define debug(x) #x << "=" << (x)

#ifdef DEBUG
#define _GLIBCXX_DEBUG
#define print(x) std::cerr << debug(x) << " (L:" << __LINE__ << ")" << std::endl
#else
#define print(x)
#endif

template <i64 p>
class fp {
    public:
    i64 x;
    fp() : x(0) {}
    fp(i64 x_) : x((x_ % p + p) % p) {}
    fp operator+() const { return fp(x); }
    fp operator-() const { return fp(-x); }
    fp& operator+=(const fp& y) {
        x += y.x;
        if (x >= p) x -= p;
        return *this;
    }
    fp& operator-=(const fp& y) { return *this += -y; }
    fp& operator*=(const fp& y) {
        x = x * y.x % p;
        return *this;
    }
    fp& operator/=(const fp& y) { return *this *= fp(inverse(y.x)); }
    fp operator+(const fp& y) const { return fp(x) += y; }
    fp operator-(const fp& y) const { return fp(x) -= y; }
    fp operator*(const fp& y) const { return fp(x) *= y; }
    fp operator/(const fp& y) const { return fp(x) /= y; }
    bool operator==(const fp& y) const { return x == y.x; }
    bool operator!=(const fp& y) const { return !(*this == y); }
    i64 extgcd(i64 a, i64 b, i64& x, i64& y) {
        i64 d = a;
        if (b != 0) {
            d = extgcd(b, a % b, y, x);
            y -= (a / b) * x;
        } else {
            x = 1;
            y = 0;
        }
        return d;
    }
    i64 inverse(i64 a) {
        i64 x, y;
        extgcd(a, p, x, y);
        return (x % p + p) % p;
    }
};

template <i64 p>
i64 abs(const fp<p>& x) { return x.x; }

template <i64 p>
istream& operator>>(istream& is, fp<p>& x) {
    is >> x.x;
    return is;
}

template <i64 p>
ostream& operator<<(ostream& os, const fp<p>& x) {
    os << x.x;
    return os;
}

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

template <typename T>
ostream& operator<<(ostream& os, const vector<T>& vec) {
    os << "[";
    for (const auto& v : vec) {
        os << v << ",";
    }
    os << "]";
    return os;
}

template <typename T>
bool chmin(T& a, const T& b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}

template <typename T>
bool chmax(T& a, const T& b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}

template <typename A, typename T, size_t size>
void fill(A (&ary)[size], const T& val) {
    fill((T*)ary, (T*)(ary + size), val);
}

// x未満の要素の個数を返す
template <typename T>
i64 count_less(const vector<T>& v, T x) {
    return lower_bound(begin(v), end(v), x) - begin(v);
}

// x以下の要素の個数を返す
template <typename T>
i64 count_less_or_equal(const vector<T>& v, T x) {
    return upper_bound(begin(v), end(v), x) - begin(v);
}

// xより大きい要素の個数を返す
template <typename T>
i64 count_greater(const vector<T>& v, T x) {
    return end(v) - upper_bound(begin(v), end(v), x);
}

// x以上の要素の個数を返す
template <typename T>
i64 count_greater_or_equal(const vector<T>& v, T x) {
    return end(v) - lower_bound(begin(v), end(v), x);
}

// x未満の要素のうち最大のものを返す
template <typename T>
T max_less(const vector<T>& v, T x) {
    return *(lower_bound(begin(v), end(v), x) - 1);
}

// x以下の要素のうち最大のものを返す
template <typename T>
T max_less_or_equal(const vector<T>& v, T x) {
    return *(upper_bound(begin(v), end(v), x) - 1);
}

// xより大きい要素のうち最小のものを返す
template <typename T>
T min_greater(const vector<T>& v, T x) {
    return *upper_bound(begin(v), end(v), x);
}

// x以上の要素のうち最小のものを返す
template <typename T>
T min_greater_or_equal(const vector<T>& v, T x) {
    return *lower_bound(begin(v), end(v), x);
}

constexpr int inf = 1.01e9;
constexpr i64 inf64 = 4.01e18;
constexpr long double eps = 1e-9;

// 実数値のn分探索のループ回数の上限(2分探索なら50でも十分かもしれない)
constexpr i64 max_loop = 100;

long double tolerance(const long double& a, const long double& b, const long double& epsilon = eps) {
    return epsilon * max({1.l, abs(a), abs(b)});
}

// a<0なら-1, a==0なら0, a>0なら1
int sgn(const long double a, const long double& epsilon = eps) {
    return (a > epsilon) - (a < -epsilon);
}

// a<bなら-1, a==bなら0, a>bなら1
int compare(const long double& a, const long double& b, const long double& epsilon = eps, bool is_relative = true) {
    return sgn(a - b, is_relative ? tolerance(a, b, epsilon) : epsilon);
}

// a==bか?
bool is_equal(const long double& a, const long double& b, const long double& epsilon = eps, bool is_relative = true) {
    return compare(a, b, epsilon, is_relative) == 0;
}

// a<bか?
bool is_less(const long double& a, const long double& b, const long double& epsilon = eps, bool is_relative = true) {
    return compare(a, b, epsilon, is_relative) == -1;
}

// a<=bか?
bool is_less_or_equal(const long double& a, const long double& b, const long double& epsilon = eps, bool is_relative = true) {
    return compare(a, b, epsilon, is_relative) <= 0;
}

// a>bか?
bool is_greater(const long double& a, const long double& b, const long double& epsilon = eps, bool is_relative = true) {
    return compare(a, b, epsilon, is_relative) == 1;
}

// a>=bか?
bool is_greater_or_equal(const long double& a, const long double& b, const long double& epsilon = eps, bool is_relative = true) {
    return compare(a, b, epsilon, is_relative) >= 0;
}

void solve() {
    //constexpr i64 mod = 1'000'000'007;
    i64 N,M;
    cin >> N >> M;

    vector<set<i64>> in(N),out(N);
    rep(i,0,M){
        i64 A,B;
        cin >> A >> B;
        --A;
        --B;
        out[A].insert(B);
        in[B].insert(A);
    }

    vector<i64> state(N); // 0: 未到達, 1: 探索中, 2: 終了済み
    vector<i64> to(N),cycle;
    function<bool(i64)> dfs=[&](i64 u){
        state[u]=1;
        for(auto v : out[u]){
            if(state[v]==2) continue;

            to[u]=v;
            if(state[v]==1){
                i64 w=v;
                do{
                    cycle.emplace_back(w);
                    w=to[w];
                }while(w!=v);
                return true;
            }

            if(dfs(v)) return true;
        }
        state[u]=2;
        return false;
    };

    rep(u,0,N){
        if(state[u]!=0) continue;
        if(dfs(u)) break;
    }

    if(cycle.empty()){
        cout << -1 << endl;
        return;
    }

    print(cycle);
    vector<i64> id(N,-1);
    rep(i,0,cycle.size()){
        id[cycle[i]]=i;
    }

    set<i64> removed;
    rep(i,0,N){
        if(id[i]==-1) removed.insert(i);
    }

    auto remove=[&]{
        for(auto u:removed){
            for(auto v:out[u]){
                in[v].erase(u);
            }
            for(auto v:in[u]){
                out[v].erase(u);
            }
        }
        removed.clear();
    };

    remove();

    while(true){
        bool ok=true;
        for(auto u:cycle){
            if(in[u].size()==1 and out[u].size()==1) continue;
            ok=false;
            break;
        }
        if(ok) break;


        for(auto u:cycle){
            if(out[u].size()==1) continue;

            i64 v=-1;
            for(auto w:out[u]){
                if(to[u]==w) continue;
                v=w;
                break;
            }
            assert(v!=-1);

            i64 w=u;
            while(true){
                w=to[w];
                if(w==v) break;
                removed.insert(w);
            }

            to[u]=v;
            break;
        }

        for(auto u:removed){
            cycle.erase(find(begin(cycle),end(cycle),u));
        }

        remove();
    }

    cout << cycle.size() << endl;

    rep(i,0,cycle.size()){
        cout << cycle[i]+1 << endl;
    }
}

int main() {
    std::cin.tie(0);
    std::ios::sync_with_stdio(false);
    cout.setf(ios::fixed);
    cout.precision(16);
    solve();
    return 0;
}


Measured memory_bytes: 640