#include <bits/stdc++.h>
#include <boost/algorithm/string/join.hpp>
#include <boost/range/irange.hpp>
#include <boost/multiprecision/cpp_int.hpp>
using namespace std;
using boost::irange;
using boost::algorithm::join;

typedef long long ll;
typedef unsigned long long ull;
//typedef long double ld;

typedef map<int, int> mii;
typedef pair<int, int> pii;
typedef pair<double,double> pdd;
typedef set<int> si;
typedef set<ll> sll;
typedef vector<int> vi;
typedef vector<pair<int, int > > vii;
typedef vector<ll> vll;
typedef vector<double> vd;
typedef vector<vi> vvi;
typedef vector<vii> vvii;
typedef vector<vll> vvll;

constexpr ll MOD = 1e9+7;
template<typename T>constexpr auto inf=numeric_limits<T>::max()/4;
constexpr int INF=inf<int>;
constexpr ll LINF=inf<ll>;

#define Sort(x) sort(begin(x),end(x))
#define Reverse(x) reverse(begin(x),end(x))
#define ABS(a,b) ((a)<(b)?(b)-(a):(a)-(b))
#define nbit(n) (1ull<<(n))
#define _ol3(_1,_2,_3,name,...)name
#define _rep(i,n)_repi(i,0,n)
#define _repi(i,a,b)for(int i=a,i##_l=(b);i<i##_l;++i)
#define REP(...) _ol3(__VA_ARGS__,_repi,_rep,)(__VA_ARGS__)
#define REPR(i,n) for(int i=n-1;i>=0;--i)
#define REPA(i,v) REP(i,(v).size())

// sort(begin(x), end(x), greater<ll>());
// lower_bound(a.begin(), a.end(), val): a[i] ≥ valである最左の位置
// upper_bound(a.begin(), a.end(), val): a[i] > valである最左の位置

// Returns minimum of a and b.
// If a is less b, a is set to b.
template<typename T>
T chmin(T& a, T b) {
  if (a > b) {
    a = b;
  }
  return a;
}

// Returns maximum of a and b.
// If a is less b, a is set to b.
template<typename T>
T chmax(T& a, T b) {
  if (a < b) {
    a = b;
  }
  return a;
}

template <typename T>
void dumpContents(const T& v, const string& msg)
{
  cerr << "### " << msg << " ###\n";
  for (const auto& x : v) {
    cerr << x << " ";
  }
  cerr << endl;
}

struct before_main_function {
  before_main_function() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout<<setprecision(20)<<fixed;
    #define endl "\n"
  }
} before_main_function;

// ========== end of template ==========

constexpr int N_MAX = 1000;
int N, M;
vi G[N_MAX+1];

constexpr int NOV    = 0;
constexpr int OPEN   = 1;
constexpr int CLOSED = 2;
vi state(N_MAX+1, NOV);

vi vs; // stuck for keeping the current path

bool dfs(int u) {
  state[u] = OPEN;
  vs.push_back(u);

  for (int v : G[u]) {
    if (state[v] == OPEN) {
      // found a cycle : v -> u
      vs.erase(begin(vs), find(begin(vs), end(vs), v));
      return true;
    }
    if (state[v] == NOV && dfs(v)) {
      return true;
    }
  }

  state[u] = CLOSED;
  vs.pop_back();

  return false;
}

void solve() {
  vi visited(N), nxt(N, -1), erased(N,1);

  REP(i, vs.size()) {
    int u = vs[i];
    erased[u]=0;

    int v = vs[(i+1)%vs.size()];
    nxt[u] = v;
  }

  int u = vs[0];
  while (!visited[u]) {
    for (const int v : G[u]) {
      if (erased[v]) continue;

      int x = nxt[u];
      while (x != v) {
	erased[x] = 1;
        x = nxt[x];
      }

      nxt[u] = v;
    }
    visited[u] = true;
    u = nxt[u];
  }

  vi ret;
  const int start = u;
  while (true) {
    ret.push_back(u);
    u = nxt[u];
    if (u==start) break;
  }

  cout << ret.size() << endl;
  for(int x : ret) {
    cout << x+1 << endl;
  }
}

int main(int argc, char** argv) {
  cin >> N >> M;

  REP(i, M) {
    int A, B;
    cin >> A >> B;
    A--; B--;

    G[A].emplace_back(B);
  }

  REP(i, N) {
    if (state[i]) continue;

    if (dfs(i)) {
//dumpContents(vs, "vs");
      solve();
      return 0;
    }
  }

  cout << -1 << endl;
  return 0;
}


Measured memory_bytes: 384