///////////////////////////////////////////////////////////////////////////////

#include <bits/stdc++.h>
#include <unistd.h>
#include <time.h>
#include <sys/time.h>

using namespace std;

///////////////////////////////////////////////////////////////////////////////

#define DEBUG 0

#define pb push_back
#define V vector
#define M unordered_map
#define S static

#define rep(i,n) for(ll i=0LL; i<n; ++i)
#define srep(i,s,n) for(ll i=s; i<n; ++i)
#define rrep(i,n) for(ll i=n-1LL; i>=0LL; --i)
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(),(a).rend()
#define CIN(x) do { \
        assert(!cin.eof()); \
        cin >> x; \
        assert(!cin.fail()); \
} while(0);
#if DEBUG
#define debug_print(...) _debug_print(__VA_ARGS__)
#define debug_printf(...) printf(__VA_ARGS__)
#define debug_print_time _debug_print_time
#else // DEBUG
#define debug_print(...)
#define debug_printf(...)
#define debug_print_time
#endif // DEBUG

typedef long long ll;
typedef unsigned long long ull;
typedef tuple<ll, ll> t2;
typedef tuple<ll, ll, ll> t3;
typedef tuple<ll, ll, ll, ll> t4;
typedef tuple<ll, ll, ll, ll, ll> t5;
template <typename T>
using priority_queue_incr = priority_queue<T, V<T>, greater<T> >;

///////////////////////////////////////////////////////////////////////////////

template <typename TT, typename T>
T get_m(M<TT, T> &m, TT k, T default_value)
{
    if (m.find(k) == m.end()) return m[k] = default_value;
    return m[k];
}

template <typename TT, typename T>
void incr_m(M<TT, T> &m, TT k)
{
    if (m.find(k) == m.end()) m[k] = 0;
    m[k]++;
}

template <typename TT, typename T>
void incr_m(map<TT, T> &m, TT k)
{
    if (m.find(k) == m.end()) m[k] = 0;
    m[k]++;
}

void llin(ll &a)
{
    CIN(a);
}

void llinl1(V<ll> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a;
        CIN(a);
        v.push_back(a);
    }
}

void llinl2(V<t2> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a, b;
        CIN(a >> b);
        v.push_back(t2(a, b));
    }
}

void llinl3(V<t3> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a, b, c;
        CIN(a >> b >> c);
        v.push_back(t3(a, b, c));
    }
}

void llinl4(V<t4> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a, b, c, d;
        CIN(a >> b >> c >> d);
        v.push_back(t4(a, b, c, d));
    }
}

void llina(V<ll> &v, ll count)
{
    llinl1(v, count);
}

template <typename T>
void sort(V<T> &v)
{
    sort(v.begin(), v.end());
}

template <typename T>
void sort_reverse(V<T> &v)
{
    sort(v.begin(), v.end(), greater<T>());
}

template <typename T>
void _debug_print(T x)
{
    cout << x << endl;
}

void _debug_print(const t2 &x)
{
    ll x1 = get<0>(x);
    ll x2 = get<1>(x);
    cout << "-- " << x1 << " -- " << x2 << endl;
}

void _debug_print(const t3 &x)
{
    ll x1 = get<0>(x);
    ll x2 = get<1>(x);
    ll x3 = get<2>(x);
    cout << "-- " << x1 << " -- " << x2 << " -- " << x3 << endl;
}

void _debug_print(const t4 &x)
{
    ll x1 = get<0>(x);
    ll x2 = get<1>(x);
    ll x3 = get<2>(x);
    ll x4 = get<3>(x);
    cout << "-- " << x1 << " -- " << x2
         << " -- " << x3 << " -- " << x4 << endl;
}

template <typename T>
void _debug_print(T xarray[], ll n)
{
    rep (i, n) _debug_print(xarray[i]);
}

template <typename T>
void _debug_print(const V<T> &xlist)
{
    for (auto x : xlist) {
        cout << "-- ";
        _debug_print(x);
    }
}

template <typename T>
void _debug_print(const set<T> &xset)
{
    for (auto x : xset) {
        cout << "-- ";
        _debug_print(x);
    }
}

template <typename TT, typename T>
void _debug_print(const M<TT, T> &xlist)
{
    for (auto x : xlist) {
        TT k = x.first;
        T v = x.second;
        cout << "====" << endl;
        cout << "K=";
        _debug_print(k);
        cout << "V=";
        _debug_print(v);
    }
}

template <typename TT, typename T>
void _debug_print(const map<TT, T> &xlist)
{
    for (auto x : xlist) {
        TT k = x.first;
        T v = x.second;
        cout << "====" << endl;
        cout << "K=";
        _debug_print(k);
        cout << "V=";
        _debug_print(v);
    }
}

void _debug_print_time(const char *prefix)
{
    struct timeval tv;
    gettimeofday(&tv, NULL);
#if DEBUG
    struct tm *tm = localtime(&tv.tv_sec);
#endif
    debug_printf("-- %s %02d:%02d:%02d.%06ld\n",
                 prefix, tm->tm_hour, tm->tm_min, tm->tm_sec, tv.tv_usec);
}

///////////////////////////////////////////////////////////////////////////////

void _main();
int main()
{
    cout << setprecision(12);
#if !DEBUG
    ios::sync_with_stdio(false);
    cin.tie(0);
#endif

    _main();
    return 0;
}

void make_edges(V<ll> edges[], V<ll> redges[], const V<t2> &uvlist)
{
    for (auto uv : uvlist) {
        ll u = get<0>(uv);
        ll v = get<1>(uv);
        edges[u].pb(v);
        redges[v].pb(u);
    }
}

bool solve(ll sv, ll v, const V<ll> nedges[], set<ll> &path, bool checked[])
{
    if (sv == 2) {
        debug_printf("---- solve sv=%lld v=%lld\n", sv, v);
        debug_printf("---- path\n");
        debug_print(path);
    }

    if (path.find(v) != path.end()) {
        if (sv == v) {
            if (sv == 2) debug_printf("---- xx1\n");
            map<ll, ll> cnts_out;
            map<ll, ll> cnts_in;
            for (auto v : path) for (auto nv : nedges[v]) {
                    if (path.find(nv) == path.end()) continue;
                    incr_m(cnts_out, v);
                    incr_m(cnts_in, nv);
                }
            if (sv == 2) {
                debug_printf("---- cnts_out\n");
                debug_print(cnts_out);
                debug_printf("---- cnts_in\n");
                debug_print(cnts_in);
            }
            for (auto v : path) {
                if (cnts_out.find(v) == cnts_out.end()) return false;
                if (cnts_in.find(v) == cnts_in.end()) return false;
                if (cnts_out[v] > 1) return false;
                if (cnts_in[v] > 1) return false;
            }
            debug_printf("----------- ans\n");
            cout << path.size() << endl;
            for (auto v : path) cout << v << endl;
            return true;
        }
        return false;
    }

    if (checked[v]) return false;

    path.insert(v);
    checked[v] = true;

    for (auto nv : nedges[v]) {
        if (solve(sv, nv, nedges, path, checked)) return true;
    }

    path.erase(v);

    return false;
}

void _main()
{
    ll n; llin(n);
    ll m; llin(m);
    V<t2> uvlist; llinl2(uvlist, m);

    V<ll> edges[1005];
    V<ll> redges[1005];
    make_edges(edges, redges, uvlist);

    ll cnts_out[1005] = {};
    ll cnts_in[1005] = {};
    for (auto uv : uvlist) {
        ll u = get<0>(uv);
        ll v = get<1>(uv);
        cnts_out[u]++;
        cnts_in[v]++;
    }

    queue<ll> q;
    bool removed[1005] = {};

    srep (v, 1, n+1) if (!cnts_in[v]) q.push(v);
    while (q.size()) {
        ll v = q.front(); q.pop();
        removed[v] = true;
        for (auto nv : edges[v]) {
            cnts_in[nv]--;
            if (!cnts_in[nv]) q.push(nv);
        }
    }

    srep (v, 1, n+1) if (!cnts_out[v]) q.push(v);
    while (q.size()) {
        ll v = q.front(); q.pop();
        removed[v] = true;
        for (auto nv : redges[v]) {
            cnts_out[nv]--;
            if (!cnts_out[nv]) q.push(nv);
        }
    }

    debug_printf("---- removed\n");
    debug_print(removed, n+1);

    ll ncnts_out[1005] = {};
    ll ncnts_in[1005] = {};
    set<ll> vset;
    for (auto uv : uvlist) {
        ll u = get<0>(uv);
        ll v = get<1>(uv);
        if (removed[u]) continue;
        if (removed[v]) continue;
        ncnts_out[u]++;
        ncnts_in[v]++;
        vset.insert(u);
        vset.insert(v);
    }

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

    S V<ll> nedges[1005];
    for (auto uv : uvlist) {
        ll u = get<0>(uv);
        ll v = get<1>(uv);
        if (removed[u]) continue;
        if (removed[v]) continue;
        nedges[u].pb(v);
    }

    srep (v, 1, n+1) {
        bool checked[1005] = {};
        set<ll> path;
        if (solve(v, v, nedges, path, checked)) return;
    }

    cout << -1 << endl;
}

///////////////////////////////////////////////////////////////////////////////


Measured memory_bytes: 4328