Problem p02902 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
// using namespace std;
#pragma GCC target("avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#define rep(i, j, n) for (ll i = (ll)(j); i < (ll)(n); i++)
#define REP(i, j, n) for (ll i = (ll)(j); i <= (ll)(n); i++)
#define per(i, j, n) for (ll i = (ll)(j); (ll)(n) <= i; i--)
#define ll long long
#define ALL(a) (a).begin(), (a).end()
#define disup(A, key) distance(A.begin(), upper_bound(ALL(A), (ll)(key)))
#define dislow(A, key) distance(A.begin(), lower_bound(ALL(A), (ll)(key)))
#define pb emplace_back
#define mp std::make_pair
// #define endl "\n"
//
using std::cin;
using std::cout;
using std::endl;
using std::lower_bound;
using std::string;
using std::upper_bound;
using std::vector;
using vi = vector<ll>;
using vii = vector<vi>;
using pii = std::pair<ll, ll>;
// constexpr
ll MOD = 1e9 + 7;
// constexpr ll MOD=998244353;
// constexpr ll MOD=10000000;
constexpr ll MAX = 1e6;
constexpr ll INF = (1ll << 62);
template <class T>
class prique : public std::priority_queue<T, std::vector<T>, std::greater<T>> {
};
struct Segment_tree {
  ll N;
  vector<pii> node;
  Segment_tree(ll sz) {
    N = 1;
    while (N < sz)
      N *= 2;
    node.resize(2 * N - 1, mp(INF, INF));
    per(i, N - 2, 0) { node[i] = std::min(node[i * 2 + 1], node[i * 2 + 1]); }
  }
  void update(ll X, pii val) {
    X += N - 1;
    node[X] = val;
    while (X > 0) {
      X = (X - 1) / 2;
      node[X] = std::min(node[X * 2 + 1], node[X * 2 + 2]);
    }
  }
  pii RMQ(ll a, ll b, ll now, ll l, ll r) { //[a,b),[l,r)
    if (r < 0)
      r = N;
    if (r <= a || b <= l)
      return mp(INF, INF);
    if (a <= l && r <= b)
      return node[now];
    auto vl = RMQ(a, b, now * 2 + 1, l, (l + r) / 2),
         vr = RMQ(a, b, now * 2 + 2, (l + r) / 2, r);
    return std::min(vl, vr);
  }
};
struct Binary_indexed_tree {
  int N;
  vi bit;
  Binary_indexed_tree(int n) : N(n) { bit.resize(N + 1, 0); }
  void add(int x, int a) {
    for (x; x <= N; x += (x & -x))
      bit[x] += a;
  }
  ll sum(int x) {
    ll ret = 0;
    for (x; x > 0; x -= (x & -x))
      ret += bit[x];
    return ret;
  }
  ll lower_bound(ll X) {
    if (sum(N) < X)
      return -1;
    ll ret = 0, memo = 1, sum = 0;
    while (memo * 2 <= N)
      memo *= 2;
    while (memo > 0) {
      if (memo + ret <= N && sum + bit[memo + ret] < X) {
        sum += bit[memo + ret];
        ret += memo;
      }
      memo /= 2;
    }
    return ret + 1;
  }
};
struct Union_Find {
  ll N;
  vi par;
  vi siz;
  Union_Find(int n) : N(n) {
    par.resize(N);
    siz.resize(N, 1);
    rep(i, 0, N) par[i] = i;
  }
  ll root(ll X) {
    if (par[X] == X)
      return X;
    return par[X] = root(par[X]);
  }
  bool same(ll X, ll Y) { return root(X) == root(Y); }
  void unite(ll X, ll Y) {
    X = root(X);
    Y = root(Y);
    if (X == Y)
      return;
    par[X] = Y;
    siz[Y] += siz[X];
    siz[X] = 0;
  }
  ll size(ll X) { return siz[root(X)]; }
};
long long modpow(long long a, long long n, long long mod) {
  long long res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}
vi fac, finv, inv;
void COMinit() {
  fac.resize(MAX);
  finv.resize(MAX);
  inv.resize(MAX);
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}
ll COM(ll n, ll r) {
  if (n < r || n < 0 || r < 0)
    return 0;
  return fac[n] * finv[r] % MOD * finv[n - r] % MOD;
}
int main() {
  std::ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  ll N, M;
  cin >> N >> M;
  vii edge(N), memo(N);
  rep(i, 0, M) {
    ll X, Y;
    cin >> X >> Y;
    edge[X - 1].pb(Y - 1);
    memo[Y - 1].pb(X - 1);
  }
  ll ans = INF, s, g;
  rep(i, 0, N) {
    vi dist(N, -1);
    dist[i] = 0;
    std::queue<ll> que;
    que.push(i);
    while (!que.empty()) {
      ll X = que.front();
      que.pop();
      rep(j, 0, edge[X].size()) {
        ll Y = edge[X][j];
        if (dist[Y] == -1) {
          dist[Y] = dist[X] + 1;
          que.push(Y);
        } else if (Y == i && dist[X] + 1 < ans) {
          ans = dist[X] - dist[Y] + 1;
          s = i, g = X;
        }
      }
    }
  }
  if (ans == INF)
    cout << -1 << endl;
  else {
    cout << ans << endl;
    vi dist(N, -1);
    dist[s] = 0;
    std::queue<ll> que;
    que.push(s);
    while (!que.empty()) {
      ll X = que.front();
      que.pop();
      rep(j, 0, edge[X].size()) {
        ll Y = edge[X][j];
        if (dist[Y] == -1) {
          dist[Y] = dist[X] + 1;
          que.push(Y);
        }
      }
    }
    ll X = g;
    cout << g + 1 << endl;
    while (1) {
      rep(i, 0, memo[X].size()) {
        ll Y = memo[X][i];
        if (dist[Y] == dist[X] - 1) {
          cout << Y + 1 << endl;
          X = Y;
        }
      }
      if (X == s)
        break;
    }
  }
}


Fixed submission:
#include <bits/stdc++.h>
// using namespace std;
#pragma GCC target("avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#define rep(i, j, n) for (ll i = (ll)(j); i < (ll)(n); i++)
#define REP(i, j, n) for (ll i = (ll)(j); i <= (ll)(n); i++)
#define per(i, j, n) for (ll i = (ll)(j); (ll)(n) <= i; i--)
#define ll long long
#define ALL(a) (a).begin(), (a).end()
#define disup(A, key) distance(A.begin(), upper_bound(ALL(A), (ll)(key)))
#define dislow(A, key) distance(A.begin(), lower_bound(ALL(A), (ll)(key)))
#define pb emplace_back
#define mp std::make_pair
// #define endl "\n"
//
using std::cin;
using std::cout;
using std::endl;
using std::lower_bound;
using std::string;
using std::upper_bound;
using std::vector;
using vi = vector<ll>;
using vii = vector<vi>;
using pii = std::pair<ll, ll>;
// constexpr
ll MOD = 1e9 + 7;
// constexpr ll MOD=998244353;
// constexpr ll MOD=10000000;
constexpr ll MAX = 1e6;
constexpr ll INF = (1ll << 62);
template <class T>
class prique : public std::priority_queue<T, std::vector<T>, std::greater<T>> {
};
struct Segment_tree {
  ll N;
  vector<pii> node;
  Segment_tree(ll sz) {
    N = 1;
    while (N < sz)
      N *= 2;
    node.resize(2 * N - 1, mp(INF, INF));
    per(i, N - 2, 0) { node[i] = std::min(node[i * 2 + 1], node[i * 2 + 1]); }
  }
  void update(ll X, pii val) {
    X += N - 1;
    node[X] = val;
    while (X > 0) {
      X = (X - 1) / 2;
      node[X] = std::min(node[X * 2 + 1], node[X * 2 + 2]);
    }
  }
  pii RMQ(ll a, ll b, ll now, ll l, ll r) { //[a,b),[l,r)
    if (r < 0)
      r = N;
    if (r <= a || b <= l)
      return mp(INF, INF);
    if (a <= l && r <= b)
      return node[now];
    auto vl = RMQ(a, b, now * 2 + 1, l, (l + r) / 2),
         vr = RMQ(a, b, now * 2 + 2, (l + r) / 2, r);
    return std::min(vl, vr);
  }
};
struct Binary_indexed_tree {
  int N;
  vi bit;
  Binary_indexed_tree(int n) : N(n) { bit.resize(N + 1, 0); }
  void add(int x, int a) {
    for (x; x <= N; x += (x & -x))
      bit[x] += a;
  }
  ll sum(int x) {
    ll ret = 0;
    for (x; x > 0; x -= (x & -x))
      ret += bit[x];
    return ret;
  }
  ll lower_bound(ll X) {
    if (sum(N) < X)
      return -1;
    ll ret = 0, memo = 1, sum = 0;
    while (memo * 2 <= N)
      memo *= 2;
    while (memo > 0) {
      if (memo + ret <= N && sum + bit[memo + ret] < X) {
        sum += bit[memo + ret];
        ret += memo;
      }
      memo /= 2;
    }
    return ret + 1;
  }
};
struct Union_Find {
  ll N;
  vi par;
  vi siz;
  Union_Find(int n) : N(n) {
    par.resize(N);
    siz.resize(N, 1);
    rep(i, 0, N) par[i] = i;
  }
  ll root(ll X) {
    if (par[X] == X)
      return X;
    return par[X] = root(par[X]);
  }
  bool same(ll X, ll Y) { return root(X) == root(Y); }
  void unite(ll X, ll Y) {
    X = root(X);
    Y = root(Y);
    if (X == Y)
      return;
    par[X] = Y;
    siz[Y] += siz[X];
    siz[X] = 0;
  }
  ll size(ll X) { return siz[root(X)]; }
};
long long modpow(long long a, long long n, long long mod) {
  long long res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % mod;
    a = a * a % mod;
    n >>= 1;
  }
  return res;
}
vi fac, finv, inv;
void COMinit() {
  fac.resize(MAX);
  finv.resize(MAX);
  inv.resize(MAX);
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}
ll COM(ll n, ll r) {
  if (n < r || n < 0 || r < 0)
    return 0;
  return fac[n] * finv[r] % MOD * finv[n - r] % MOD;
}
int main() {
  std::ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  ll N, M;
  cin >> N >> M;
  vii edge(N), memo(N);
  rep(i, 0, M) {
    ll X, Y;
    cin >> X >> Y;
    edge[X - 1].pb(Y - 1);
    memo[Y - 1].pb(X - 1);
  }
  ll ans = INF, s, g;
  rep(i, 0, N) {
    vi dist(N, -1);
    dist[i] = 0;
    std::queue<ll> que;
    que.push(i);
    while (!que.empty()) {
      ll X = que.front();
      que.pop();
      rep(j, 0, edge[X].size()) {
        ll Y = edge[X][j];
        if (dist[Y] == -1) {
          dist[Y] = dist[X] + 1;
          que.push(Y);
        } else if (Y == i && dist[X] + 1 < ans) {
          ans = dist[X] - dist[Y] + 1;
          s = i, g = X;
        }
      }
    }
  }
  if (ans == INF)
    cout << -1 << endl;
  else {
    cout << ans << endl;
    vi dist(N, -1);
    dist[s] = 0;
    std::queue<ll> que;
    que.push(s);
    while (!que.empty()) {
      ll X = que.front();
      que.pop();
      rep(j, 0, edge[X].size()) {
        ll Y = edge[X][j];
        if (dist[Y] == -1) {
          dist[Y] = dist[X] + 1;
          que.push(Y);
        }
      }
    }
    ll X = g;
    cout << g + 1 << endl;
    while (1) {
      rep(i, 0, memo[X].size()) {
        ll Y = memo[X][i];
        if (dist[Y] == dist[X] - 1) {
          cout << Y + 1 << endl;
          X = Y;
          break;
        }
      }
      if (X == s)
        break;
    }
  }
}