#ifdef stderr_path
#define LOCAL
#endif
#ifdef LOCAL
#define _GLIBCXX_DEBUG
#else
#pragma GCC optimize("Ofast")
#endif
#include <algorithm>
#include <bitset>
#include <cassert>
#include <chrono>
#include <complex>
#include <cstring>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <unordered_map>
#include <unordered_set>

// #define NDEBUG
#define debug_stream std::cerr
#define iostream_untie true
#define __precision__ 10

#define all(v) std::begin(v), std::end(v)
#define rall(v) std::rbegin(v), std::rend(v)
#define __odd(n) ((n)&1)
#define __even(n) (not __odd(n))
#define __popcount(n) __builtin_popcountll(n)
#define __clz32(n) __builtin_clz(n)
#define __clz64(n) __builtin_clzll(n)
#define __ctz32(n) __builtin_ctz(n)
#define __ctz64(n) __builtin_ctzll(n)

using i64 = int_fast64_t;
using pii = std::pair<int, int>;
using pll = std::pair<int_fast64_t, int_fast64_t>;
template <class T>
using heap = std::priority_queue<T>;
template <class T>
using minheap = std::priority_queue<T, std::vector<T>, std::greater<T>>;
template <class T>
constexpr T inf = std::numeric_limits<T>::max() / T(2) - T(1123456);

namespace execution
{
    std::chrono::system_clock::time_point start_time, end_time;
    void print_elapsed_time()
    {
        end_time = std::chrono::system_clock::now();
        std::cerr << "\n----- Exec time : ";
        std::cerr << std::chrono::duration_cast<std::chrono::milliseconds>(
                         end_time - start_time)
                         .count();
        std::cerr << " ms -----\n\n";
    }
    struct setupper
    {
        setupper()
        {
            if(iostream_untie)
            {
                std::ios::sync_with_stdio(false);
                std::cin.tie(nullptr);
            }
            std::cout << std::fixed << std::setprecision(__precision__);
#ifdef stderr_path
            if(freopen(stderr_path, "a", stderr))
            {
                std::cerr << std::fixed << std::setprecision(__precision__);
            }
#endif
#ifdef stdout_path
            if(not freopen(stdout_path, "w", stdout))
            {
                freopen("CON", "w", stdout);
                std::cerr << "Failed to open the stdout file\n\n";
            }
            std::cout << "";
#endif
#ifdef stdin_path
            if(not freopen(stdin_path, "r", stdin))
            {
                freopen("CON", "r", stdin);
                std::cerr << "Failed to open the stdin file\n\n";
            }
#endif
#ifdef LOCAL
            std::cerr << "----- stderr at LOCAL -----\n\n";
            atexit(print_elapsed_time);
            start_time = std::chrono::system_clock::now();
#else
            fclose(stderr);
#endif
        }
    } __setupper;
} // namespace execution

class myclock_t
{
    std::chrono::system_clock::time_point built_pt, last_pt;
    int built_ln, last_ln;
    std::string built_func, last_func;
    bool is_built;

  public:
    explicit myclock_t() : is_built(false)
    {
    }
    void build(int crt_ln, const std::string &crt_func)
    {
        is_built = true;
        last_pt = built_pt = std::chrono::system_clock::now();
        last_ln = built_ln = crt_ln, last_func = built_func = crt_func;
    }
    void set(int crt_ln, const std::string &crt_func)
    {
        if(is_built)
        {
            last_pt = std::chrono::system_clock::now();
            last_ln = crt_ln, last_func = crt_func;
        }
        else
        {
            debug_stream << "[ " << crt_ln << " : " << crt_func << " ] "
                         << "myclock_t::set failed (yet to be built!)\n";
        }
    }
    void get(int crt_ln, const std::string &crt_func)
    {
        if(is_built)
        {
            std::chrono::system_clock::time_point crt_pt(
                std::chrono::system_clock::now());
            int64_t diff =
                std::chrono::duration_cast<std::chrono::milliseconds>(crt_pt -
                                                                      last_pt)
                    .count();
            debug_stream << diff << " ms elapsed from"
                         << " [ " << last_ln << " : " << last_func << " ]";
            if(last_ln == built_ln) debug_stream << " (when built)";
            debug_stream << " to"
                         << " [ " << crt_ln << " : " << crt_func << " ]"
                         << "\n";
            last_pt = built_pt, last_ln = built_ln, last_func = built_func;
        }
        else
        {
            debug_stream << "[ " << crt_ln << " : " << crt_func << " ] "
                         << "myclock_t::get failed (yet to be built!)\n";
        }
    }
};
#ifdef LOCAL
myclock_t __myclock;
#define build_clock() __myclock.build(__LINE__, __func__)
#define set_clock() __myclock.set(__LINE__, __func__)
#define get_clock() __myclock.get(__LINE__, __func__)
#else
#define build_clock() ((void)0)
#define set_clock() ((void)0)
#define get_clock() ((void)0)
#endif

namespace std
{
    template <class RAitr>
    void rsort(RAitr __first, RAitr __last)
    {
        sort(__first, __last, greater<>());
    }
    template <class T>
    size_t hash_combine(size_t seed, T const &key)
    {
        return seed ^ (hash<T>()(key) + 0x9e3779b9 + (seed << 6) + (seed >> 2));
    }
    template <class T, class U>
    struct hash<pair<T, U>>
    {
        size_t operator()(pair<T, U> const &pr) const
        {
            return hash_combine(hash_combine(0, pr.first), pr.second);
        }
    };
    template <class tuple_t, size_t index = tuple_size<tuple_t>::value - 1>
    struct tuple_hash_calc
    {
        static size_t apply(size_t seed, tuple_t const &t)
        {
            return hash_combine(
                tuple_hash_calc<tuple_t, index - 1>::apply(seed, t),
                get<index>(t));
        }
    };
    template <class tuple_t>
    struct tuple_hash_calc<tuple_t, 0>
    {
        static size_t apply(size_t seed, tuple_t const &t)
        {
            return hash_combine(seed, get<0>(t));
        }
    };
    template <class... T>
    struct hash<tuple<T...>>
    {
        size_t operator()(tuple<T...> const &t) const
        {
            return tuple_hash_calc<tuple<T...>>::apply(0, t);
        }
    };
    template <class T, class U>
    istream &operator>>(std::istream &s, pair<T, U> &p)
    {
        return s >> p.first >> p.second;
    }
    template <class T, class U>
    ostream &operator<<(std::ostream &s, const pair<T, U> p)
    {
        return s << p.first << " " << p.second;
    }
    template <class T>
    istream &operator>>(istream &s, vector<T> &v)
    {
        for(T &e : v)
        {
            s >> e;
        }
        return s;
    }
    template <class T>
    ostream &operator<<(ostream &s, const vector<T> &v)
    {
        bool is_front = true;
        for(const T &e : v)
        {
            if(not is_front)
            {
                s << ' ';
            }
            else
            {
                is_front = false;
            }
            s << e;
        }
        return s;
    }
    template <class tuple_t, size_t index>
    struct tupleos
    {
        static ostream &apply(ostream &s, const tuple_t &t)
        {
            tupleos<tuple_t, index - 1>::apply(s, t);
            return s << " " << get<index>(t);
        }
    };
    template <class tuple_t>
    struct tupleos<tuple_t, 0>
    {
        static ostream &apply(ostream &s, const tuple_t &t)
        {
            return s << get<0>(t);
        }
    };
    template <class... T>
    ostream &operator<<(ostream &s, const tuple<T...> &t)
    {
        return tupleos<tuple<T...>, tuple_size<tuple<T...>>::value - 1>::apply(
            s, t);
    }
    template <>
    ostream &operator<<(ostream &s, const tuple<> &t)
    {
        return s;
    }
    string revstr(string str)
    {
        reverse(str.begin(), str.end());
        return str;
    }
} // namespace std

#ifdef LOCAL
#define dump(...)                                                              \
    debug_stream << "[ " << __LINE__ << " : " << __FUNCTION__ << " ]\n",       \
        dump_func(#__VA_ARGS__, __VA_ARGS__)
template <class T>
void dump_func(const char *ptr, const T &x)
{
    debug_stream << '\t';
    for(char c = *ptr; c != '\0'; c = *++ptr)
    {
        if(c != ' ') debug_stream << c;
    }
    debug_stream << " : " << x << '\n';
}
template <class T, class... rest_t>
void dump_func(const char *ptr, const T &x, rest_t... rest)
{
    debug_stream << '\t';
    for(char c = *ptr; c != ','; c = *++ptr)
    {
        if(c != ' ') debug_stream << c;
    }
    debug_stream << " : " << x << ",\n";
    dump_func(++ptr, rest...);
}
#else
#define dump(...) ((void)0)
#endif
template <class P>
void read_range(P __first, P __second)
{
    for(P i = __first; i != __second; ++i)
        std::cin >> *i;
}
template <class P>
void write_range(P __first, P __second)
{
    for(P i = __first; i != __second;
        std::cout << (++i == __second ? '\n' : ' '))
    {
        std::cout << *i;
    }
}

// substitute y for x.
template <class T>
void subst(T &x, const T &y)
{
    x = y;
}
// substitue y for x iff x > y.
template <class T>
bool chmin(T &x, const T &y)
{
    return x > y ? x = y, true : false;
}
// substitue y for x iff x < y.
template <class T>
bool chmax(T &x, const T &y)
{
    return x < y ? x = y, true : false;
}
template <class T>
constexpr T minf(const T &x, const T &y)
{
    return std::min(x, y);
}
template <class T>
constexpr T maxf(const T &x, const T &y)
{
    return std::max(x, y);
}
// binary search.
template <class int_t, class F>
int_t bin(int_t ok, int_t ng, const F &f)
{
    while(std::abs(ok - ng) > 1)
    {
        int_t mid = (ok + ng) / 2;
        (f(mid) ? ok : ng) = mid;
    }
    return ok;
}
// be careful that val is type-sensitive.
template <class T, class A, size_t N>
void init(A (&array)[N], const T &val)
{
    std::fill((T *)array, (T *)(array + N), val);
}
void reset()
{
}
template <class A, class... rest_t>
void reset(A &array, rest_t... rest)
{
    memset(array, 0, sizeof(array));
    reset(rest...);
}
// a integer uniformly and randomly chosen from the interval [l, r).
template <typename int_t>
int_t rand_int(int_t l, int_t r)
{
    static std::random_device seed_gen;
    static std::mt19937 engine(seed_gen());
    std::uniform_int_distribution<int_t> unid(l, r - 1);
    return unid(engine);
}
// a real number uniformly and randomly chosen from the interval [l, r).
template <typename real_t>
real_t rand_real(real_t l, real_t r)
{
    static std::random_device seed_gen;
    static std::mt19937 engine(seed_gen());
    std::uniform_real_distribution<real_t> unid(l, r);
    return unid(engine);
}

/* The main code follows. */

class Strongly_connected_components
{
    const size_t V;
    std::vector<std::vector<size_t>> adj, mbr;
    std::vector<size_t> comp, low;
    size_t n;
    bool is_built;

  public:
    Strongly_connected_components(size_t _V)
        : V(_V), adj(V), comp(V), low(V), is_built()
    {}

    // add an edge from the vertex s to the vertex t.
    void add_edge(size_t s, size_t t)
    {
        adj[s].emplace_back(t);
        is_built = false;
    }

    // the number of the components.
    size_t count()
    {
        if(not is_built) build();
        return n;
    }

    // the number of vertices in the i-th component.
    size_t size(size_t i)
    {
        if(not is_built) build();
        return mbr[i].size();
    }

    // vertices in the i-th component.
    const std::vector<size_t> &component(size_t i)
    {
        if(not is_built) build();
        return mbr[i];
    }

    // the component which the vertex v belongs to.
    size_t operator[](size_t v)
    {
        if(not is_built) build();
        return comp[v];
    }

    // the directed acyclic graph consisting of the components.
    std::vector<std::vector<size_t>> make_dag()
    {
        if(not is_built) build();
        std::vector<std::vector<size_t>> ret(n);
        for(size_t i = 0; i < n; ++i)
        {
            std::unordered_set<size_t> uni;
            for(size_t s : mbr[i])
            {
                for(size_t t : adj[s])
                {
                    if(comp[t] != i) uni.emplace(comp[t]);
                }
            }
            ret[i] = std::vector<size_t>(uni.begin(), uni.end());
        }
        return ret;
    }

  protected:
    void build()
    {
        is_built = true, n = 0;
        fill(low.begin(), low.end(), -1);
        fill(comp.begin(), comp.end(), -1);
        std::vector<size_t> stack(V);
        auto itr = stack.begin();
        for(size_t v = 0, c = 0; v < V; ++v)
            affix(v, c, itr);
        mbr.resize(n);
        for(size_t v = 0; v < V; ++v)
        {
            comp[v] = n - 1 - comp[v];
            mbr[comp[v]].emplace_back(v);
        }
    }

    size_t affix(size_t v, size_t &c, std::vector<size_t>::iterator &itr)
    {
        if(~low[v]) return ~comp[v] ? -1 : low[v];
        size_t idx = c++;
        low[v] = idx;
        *itr++ = v;
        for(int u : adj[v])
        {
            low[v] = std::min(low[v], affix(u, c, itr));
        }
        if(low[v] == idx)
        {
            do
            {
                comp[*--itr] = n;
            } while(*itr != v);
            ++n;
        }
        return low[v];
    }
};




using namespace std;
// using namespace math;

signed main()
{
    void __solve();
    void __precalc();

    unsigned int t = 1;
    // cin >> t;
    __precalc();

#ifdef LOCAL
    t = 1;
#endif

    while(t--)
    {
        __solve();
    }
}

void __precalc()
{
}

bool go[1001][1010];

void __solve()
{
    int n,m; cin>>n>>m;
    vector<vector<int>> g(n);
    Strongly_connected_components scc(n);

    for(int i = 0; i < m; ++i)
    {
        int a,b; cin>>a>>b,a--,b--;
        scc.add_edge(a,b);
        g[a].emplace_back(b);
        go[a][b]=1;
    }

    int k=scc.count();
    dump(k);

    if(k==n) cout<<-1<<endl;
    else
    {
        int rep=-1;
        for(int i = 0; i < k; ++i)
        {
            if(scc.size(i)>1)
            {
                rep=i; break;
            }
        }

        dump(rep);
        assert(~rep);
        auto comp=scc.component(rep);
        int v=comp.front();

        vector<int> path(1,v);
        bool in[1<<10]={};
        for(int h : comp)
        {
            in[h]=1;
            dump(h);
        }

        bool used[1<<10]={};
        used[v]=1;
        vector<int> ans;

        while(true)
        {
            int nx=-1;
            bool flag=0;
            bool is[1<<10]={};

            for(int u : g[v])
            {
                if(not in[u]) continue;

                dump(v,u);
                if(used[u])
                {
                    flag=1;
                    is[u]=1;
                }
                else
                {
                    nx=u;
                }
            }

            if(flag)
            {
                for(auto itr=path.rbegin(); path.rend()!=itr; ++itr)
                {
                    ans.emplace_back(*itr);
                    if(is[*itr]) 
                    {
                        break;
                    }
                }
                reverse(all(ans));
                break;
            }
            else
            {
                assert(~nx);
                path.emplace_back(nx);
                used[nx]=1;
                v=nx;
            }
        } 

        int p=0;
        while(p+1<(int)ans.size())
        {
            v=ans[p];
            int nx=-1;
            int i=p+1;
            while(i< (int)ans.size())
            {
                if(go[v][ans[i]]) nx=i;
                i++;
            }
            if(1+p<nx) ans.erase(ans.begin()+p+1,ans.begin()+nx);
            p++;
        }

        assert((int)ans.size()>1);

        std::cout << ans.size() << "\n";

        {
            int ind[1<<10]={},out[1<<10]={};
            for(int i : ans)
            {
                for(int j : ans)
                {
                    if(go[i][j])
                    {
                        out[i]++;
                        in[j]++;
                    }
                }
            }

            for(int i : ans)
            {
                assert(in[i]==1 and out[i]==1);
            }
            
        }

        for(int t : ans)
        {
            std::cout << t+1 << "\n";
        }
    }
    
}


Measured memory_bytes: 1408