Problem p02991 (C++)

Buggy submission (status: Time Limit Exceeded):
#define _USE_MATH_DEFINES
#include <algorithm>
#include <cassert>
#include <climits>
#include <cmath>
#include <cstring>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <vector>
using namespace std;
#define MOD 1000000007
#define REP(i, n) for (int(i) = 0; (i) < (n); (i)++)
#define FOR(i, c)                                                              \
  for (decltype((c).begin()) i = (c).begin(); i != (c).end(); ++i)
#define ll long long
#define ull unsigned long long
#define all(hoge) (hoge).begin(), (hoge).end()
typedef pair<ll, ll> P;
const long long INF = 1LL << 60;
typedef vector<ll> Array;
typedef vector<Array> Matrix;

// priority_queue<ll> max;//大きい順
// priority_queue<ll, Array, greater<ll>> min;//小さい順

/*firstについては昇順 secondについては降順
sort(all(wh), [&](P x, P y) {
        if (x.first == y.first)return x.second > y.second;
        return x.first < y.first;
});
*/

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

// substr　文字列取り出し
// upper_bound
// ある値より大きい一番左のイテレータを返す、lowerは以上(setに対して使うとO(N)なので、setのメンバ関数を使う
// stoi

struct Edge { // グラフ
  ll to, cap, rev;
  Edge(ll _to, ll _cap, ll _rev) {
    to = _to;
    cap = _cap;
    rev = _rev;
  }
};
typedef vector<Edge> Edges;
typedef vector<Edges> Graph;
void add_edge(Graph &G, ll from, ll to, ll cap, bool revFlag,
              ll revCap) { // 最大フロー求める Ford-fulkerson
  G[from].push_back(Edge(to, cap, (ll)G[to].size()));
  if (revFlag)
    G[to].push_back(Edge(
        from, revCap, (ll)G[from].size() - 1)); // 最小カットの場合逆辺は0にする
}
void erase_edge(Graph &G, ll from, ll num) { // 使っちゃいけないタイミングあり
  swap(G[from][num], G[from][G[from].size() - 1]);
  G[from].pop_back();
}
ll max_flow_dfs(Graph &G, ll v, ll t, ll f, vector<bool> &used) {
  if (v == t)
    return f;
  used[v] = true;
  for (int i = 0; i < G[v].size(); ++i) {
    Edge &e = G[v][i];
    if (!used[e.to] && e.cap > 0) {
      ll d = max_flow_dfs(G, e.to, t, min(f, e.cap), used);
      if (d > 0) {
        e.cap -= d;
        G[e.to][e.rev].cap += d;
        return d;
      }
    }
  }
  return 0;
}
// 二分グラフの最大マッチングを求めたりも出来る　また二部グラフの最大独立集合は頂点数-最大マッチングのサイズ
ll max_flow(Graph &G, ll s, ll t) // O(V(V+E))
{
  ll flow = 0;
  for (;;) {
    vector<bool> used(G.size());
    REP(i, used.size()) used[i] = false;
    ll f = max_flow_dfs(G, s, t, INF, used);
    if (f == 0) {
      return flow;
    }
    flow += f;
  }
}
void BellmanFord(Graph &G, ll s, Array &d, Array &negative) { // O(|E||V|)
  d.resize(G.size());
  negative.resize(G.size());
  REP(i, d.size()) d[i] = INF;
  REP(i, d.size()) negative[i] = false;
  d[s] = 0;
  REP(k, G.size() - 2) {
    REP(i, G.size()) {
      REP(j, G[i].size()) {
        if (d[G[i][j].to] > d[i] + G[i][j].cap) {
          d[G[i][j].to] = d[i] + G[i][j].cap;
        }
      }
    }
  }
  REP(k, G.size() - 2) {
    REP(i, G.size()) {
      REP(j, G[i].size()) {
        if (d[G[i][j].to] > d[i] + G[i][j].cap) {
          d[G[i][j].to] = d[i] + G[i][j].cap;
          negative[G[i][j].to] = true;
        }
        if (negative[i] == true)
          negative[G[i][j].to] = true;
      }
    }
  }
}
void Dijkstra(Graph &G, ll s, Array &d) { // O(|E|log|V|)
  d.resize(G.size());
  REP(i, d.size()) d[i] = INF;
  d[s] = 0;
  priority_queue<P, vector<P>, greater<P>> q;
  q.push(make_pair(0, s));
  while (!q.empty()) {
    P a = q.top();
    q.pop();
    if (d[a.second] < a.first)
      continue;
    REP(i, G[a.second].size()) {
      Edge e = G[a.second][i];
      if (d[e.to] > d[a.second] + e.cap) {
        d[e.to] = d[a.second] + e.cap;
        q.push(make_pair(d[e.to], e.to));
      }
    }
  }
}
void WarshallFloyd(Graph &G, Matrix &d) { // O(V^3)
  d.resize(G.size());
  REP(i, d.size()) d[i].resize(G.size());
  REP(i, d.size()) {
    REP(j, d[i].size()) { d[i][j] = INF; }
  }
  REP(i, G.size()) {
    REP(j, G[i].size()) { d[i][G[i][j].to] = G[i][j].cap; }
  }
  REP(i, G.size()) {
    REP(j, G.size()) {
      REP(k, G.size()) { chmin(d[j][k], d[j][i] + d[i][k]); }
    }
  }
}

class UnionFind {
  vector<int> data;
  ll num;

public:
  UnionFind(int size) : data(size, -1), num(size) {}
  bool unionSet(int x, int y) { // xとyの集合を統合する
    x = root(x);
    y = root(y);
    if (x != y) {
      if (data[y] < data[x])
        swap(x, y);
      data[x] += data[y];
      data[y] = x;
    }
    num -= (x != y);
    return x != y;
  }
  bool findSet(int x, int y) { // xとyが同じ集合か返す
    return root(x) == root(y);
  }
  int root(int x) { // xのルートを返す
    return data[x] < 0 ? x : data[x] = root(data[x]);
  }
  int size(int x) { // xの集合のサイズを返す
    return -data[root(x)];
  }
  int numSet() { // 集合の数を返す
    return num;
  }
};
class SumSegTree {
private:
  int _sum(int a, int b, int k, int l, int r) {
    if (r <= a || b <= l)
      return 0; // 交差しない
    if (a <= l && r <= b)
      return dat[k]; // a,l,r,bの順で完全に含まれる
    else {
      int s1 = _sum(a, b, 2 * k + 1, l, (l + r) / 2); // 左の子
      int s2 = _sum(a, b, 2 * k + 2, (l + r) / 2, r); // 右の子
      return s1 + s2;
    }
  }

public:
  int n, height;
  vector<int> dat;

  // 初期化（_nは最大要素数）
  SumSegTree(int _n) {
    n = 1;
    height = 1;
    while (n < _n) {
      n *= 2;
      height++;
    }
    dat = vector<int>(2 * n - 1, 0);
  }

  // 場所i(0-indexed)にxを足す
  void add(int i, int x) {
    i += n - 1; // i番目の葉ノードへ
    dat[i] += x;
    while (i > 0) { // 下から上がっていく
      i = (i - 1) / 2;
      dat[i] += x;
    }
  }

  // 区間[a,b)の総和。ノードk=[l,r)に着目している。
  int sum(int a, int b) { return _sum(a, b, 0, 0, n); }
};
class RmqTree {
private:
  ll _find(ll a, ll b, ll k, ll l, ll r) {
    if (r <= a || b <= l)
      return INF; // 交差しない
    if (a <= l && r <= b)
      return dat[k]; // a,l,r,bの順で完全に含まれる
    else {
      ll s1 = _find(a, b, 2 * k + 1, l, (l + r) / 2); // 左の子
      ll s2 = _find(a, b, 2 * k + 2, (l + r) / 2, r); // 右の子
      return min(s1, s2);
    }
  }

public:
  ll n, height;
  vector<ll> dat;

  // 初期化（_nは最大要素数）
  RmqTree(ll _n) {
    n = 1;
    height = 1;
    while (n < _n) {
      n *= 2;
      height++;
    }
    dat = vector<ll>(2 * n - 1, INF);
  }

  // 場所i(0-indexed)をxにする
  void update(ll i, ll x) {
    i += n - 1; // i番目の葉ノードへ
    dat[i] = x;
    while (i > 0) { // 下から上がっていく
      i = (i - 1) / 2;
      dat[i] = min(dat[i * 2 + 1], dat[i * 2 + 2]);
    }
  }

  // 区間[a,b)の最小値。ノードk=[l,r)に着目している。
  ll find(ll a, ll b) { return _find(a, b, 0, 0, n); }
};

// 約数求める //約数
void divisor(ll n, vector<ll> &ret) {
  for (ll i = 1; i * i <= n; i++) {
    if (n % i == 0) {
      ret.push_back(i);
      if (i * i != n)
        ret.push_back(n / i);
    }
  }
  sort(ret.begin(), ret.end());
}

vector<ll> lis_fast(const vector<ll> &a) { // 最長部分増加列
  const ll n = a.size();
  vector<ll> A(n, INT_MAX);
  vector<ll> id(n);
  for (int i = 0; i < n; ++i) {
    id[i] = distance(A.begin(), lower_bound(A.begin(), A.end(), a[i]));
    A[id[i]] = a[i];
  }
  ll m = *max_element(id.begin(), id.end());
  vector<ll> b(m + 1);
  for (int i = n - 1; i >= 0; --i)
    if (id[i] == m)
      b[m--] = a[i];
  return b;
}

ll ModPow(ll x, ll n) {
  ll res = 1LL;
  while (n > 0) {
    if (n & 1)
      res = res * x % MOD;
    x = x * x % MOD;
    n >>= 1;
  }
  return res;
}
// nCrとか
class Combination {
public:
  Array fact;
  Array inv;
  ll mod;
  ll mod_inv(ll x) {
    ll n = mod - 2LL;
    ll res = 1LL;
    while (n > 0) {
      if (n & 1)
        res = res * x % mod;
      x = x * x % mod;
      n >>= 1;
    }
    return res;
  }
  ll nCr(ll n, ll r) { return ((fact[n] * inv[r] % mod) * inv[n - r]) % mod; }
  ll nPr(ll n, ll r) { return (fact[n] * inv[n - r]) % mod; }
  ll nHr(ll n, ll r) { return nCr(r + n - 1, r); }
  Combination(ll n, ll _mod) {
    mod = _mod;
    fact.resize(n + 1);
    fact[0] = 1;
    REP(i, n) { fact[i + 1] = (fact[i] * (i + 1LL)) % mod; }
    inv.resize(n + 1);
    REP(i, n + 1) { inv[i] = mod_inv(fact[i]); }
  }
};

ll gcd(ll m, ll n) {
  if (n == 0)
    return m;
  return gcd(n, m % n);
} // gcd

ll lcm(ll m, ll n) { return m / gcd(m, n) * n; }

Matrix mIdentity(ll n) {
  Matrix A(n, Array(n));
  for (int i = 0; i < n; ++i)
    A[i][i] = 1;
  return A;
}

Matrix mMul(const Matrix &A, const Matrix &B) {
  Matrix C(A.size(), Array(B[0].size()));
  for (int i = 0; i < C.size(); ++i)
    for (int j = 0; j < C[i].size(); ++j)
      for (int k = 0; k < A[i].size(); ++k)
        (C[i][j] += (A[i][k] % MOD) * (B[k][j] % MOD)) %= MOD;
  return C;
}
// O( n^3 log e )
Matrix mPow(const Matrix &A, ll e) {
  return e == 0       ? mIdentity(A.size())
         : e % 2 == 0 ? mPow(mMul(A, A), e / 2)
                      : mMul(A, mPow(A, e - 1));
}

Matrix used(100100, Array(3, 0));
ll cost[100100];

void dfs(Graph &graph, ll v, ll num) {
  used[v][num % 3] = num;
  if (num % 3 == 0)
    chmin(cost[v], num);
  REP(i, graph[v].size()) {
    if (used[graph[v][i].to][(num + 1) % 3] > num + 1) {
      dfs(graph, graph[v][i].to, num + 1);
    }
  }
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);

  ll n, m;
  cin >> n >> m;
  Graph graph(n);
  REP(i, m) {
    ll u, v;
    cin >> u >> v;
    u--;
    v--;
    add_edge(graph, u, v, 1, false, 0);
  }
  ll s, t;
  cin >> s >> t;
  s--;
  t--;
  REP(i, n) cost[i] = INF;
  REP(i, n) {
    REP(j, 3) { used[i][j] = INF; }
  }
  dfs(graph, s, 0);
  if (cost[t] == INF) {
    cout << -1;
  } else {
    cout << cost[t] / 3;
  }

  return 0;
}

Fixed submission:
#define _USE_MATH_DEFINES
#include <algorithm>
#include <cassert>
#include <climits>
#include <cmath>
#include <cstring>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <vector>
using namespace std;
#define MOD 1000000007
#define REP(i, n) for (int(i) = 0; (i) < (n); (i)++)
#define FOR(i, c)                                                              \
  for (decltype((c).begin()) i = (c).begin(); i != (c).end(); ++i)
#define ll long long
#define ull unsigned long long
#define all(hoge) (hoge).begin(), (hoge).end()
typedef pair<ll, ll> P;
const long long INF = 1LL << 60;
typedef vector<ll> Array;
typedef vector<Array> Matrix;

// priority_queue<ll> max;//大きい順
// priority_queue<ll, Array, greater<ll>> min;//小さい順

/*firstについては昇順 secondについては降順
sort(all(wh), [&](P x, P y) {
        if (x.first == y.first)return x.second > y.second;
        return x.first < y.first;
});
*/

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

// substr　文字列取り出し
// upper_bound
// ある値より大きい一番左のイテレータを返す、lowerは以上(setに対して使うとO(N)なので、setのメンバ関数を使う
// stoi

struct Edge { // グラフ
  ll to, cap, rev;
  Edge(ll _to, ll _cap, ll _rev) {
    to = _to;
    cap = _cap;
    rev = _rev;
  }
};
typedef vector<Edge> Edges;
typedef vector<Edges> Graph;
void add_edge(Graph &G, ll from, ll to, ll cap, bool revFlag,
              ll revCap) { // 最大フロー求める Ford-fulkerson
  G[from].push_back(Edge(to, cap, (ll)G[to].size()));
  if (revFlag)
    G[to].push_back(Edge(
        from, revCap, (ll)G[from].size() - 1)); // 最小カットの場合逆辺は0にする
}
void erase_edge(Graph &G, ll from, ll num) { // 使っちゃいけないタイミングあり
  swap(G[from][num], G[from][G[from].size() - 1]);
  G[from].pop_back();
}
ll max_flow_dfs(Graph &G, ll v, ll t, ll f, vector<bool> &used) {
  if (v == t)
    return f;
  used[v] = true;
  for (int i = 0; i < G[v].size(); ++i) {
    Edge &e = G[v][i];
    if (!used[e.to] && e.cap > 0) {
      ll d = max_flow_dfs(G, e.to, t, min(f, e.cap), used);
      if (d > 0) {
        e.cap -= d;
        G[e.to][e.rev].cap += d;
        return d;
      }
    }
  }
  return 0;
}
// 二分グラフの最大マッチングを求めたりも出来る　また二部グラフの最大独立集合は頂点数-最大マッチングのサイズ
ll max_flow(Graph &G, ll s, ll t) // O(V(V+E))
{
  ll flow = 0;
  for (;;) {
    vector<bool> used(G.size());
    REP(i, used.size()) used[i] = false;
    ll f = max_flow_dfs(G, s, t, INF, used);
    if (f == 0) {
      return flow;
    }
    flow += f;
  }
}
void BellmanFord(Graph &G, ll s, Array &d, Array &negative) { // O(|E||V|)
  d.resize(G.size());
  negative.resize(G.size());
  REP(i, d.size()) d[i] = INF;
  REP(i, d.size()) negative[i] = false;
  d[s] = 0;
  REP(k, G.size() - 2) {
    REP(i, G.size()) {
      REP(j, G[i].size()) {
        if (d[G[i][j].to] > d[i] + G[i][j].cap) {
          d[G[i][j].to] = d[i] + G[i][j].cap;
        }
      }
    }
  }
  REP(k, G.size() - 2) {
    REP(i, G.size()) {
      REP(j, G[i].size()) {
        if (d[G[i][j].to] > d[i] + G[i][j].cap) {
          d[G[i][j].to] = d[i] + G[i][j].cap;
          negative[G[i][j].to] = true;
        }
        if (negative[i] == true)
          negative[G[i][j].to] = true;
      }
    }
  }
}
void Dijkstra(Graph &G, ll s, Array &d) { // O(|E|log|V|)
  d.resize(G.size());
  REP(i, d.size()) d[i] = INF;
  d[s] = 0;
  priority_queue<P, vector<P>, greater<P>> q;
  q.push(make_pair(0, s));
  while (!q.empty()) {
    P a = q.top();
    q.pop();
    if (d[a.second] < a.first)
      continue;
    REP(i, G[a.second].size()) {
      Edge e = G[a.second][i];
      if (d[e.to] > d[a.second] + e.cap) {
        d[e.to] = d[a.second] + e.cap;
        q.push(make_pair(d[e.to], e.to));
      }
    }
  }
}
void WarshallFloyd(Graph &G, Matrix &d) { // O(V^3)
  d.resize(G.size());
  REP(i, d.size()) d[i].resize(G.size());
  REP(i, d.size()) {
    REP(j, d[i].size()) { d[i][j] = INF; }
  }
  REP(i, G.size()) {
    REP(j, G[i].size()) { d[i][G[i][j].to] = G[i][j].cap; }
  }
  REP(i, G.size()) {
    REP(j, G.size()) {
      REP(k, G.size()) { chmin(d[j][k], d[j][i] + d[i][k]); }
    }
  }
}

class UnionFind {
  vector<int> data;
  ll num;

public:
  UnionFind(int size) : data(size, -1), num(size) {}
  bool unionSet(int x, int y) { // xとyの集合を統合する
    x = root(x);
    y = root(y);
    if (x != y) {
      if (data[y] < data[x])
        swap(x, y);
      data[x] += data[y];
      data[y] = x;
    }
    num -= (x != y);
    return x != y;
  }
  bool findSet(int x, int y) { // xとyが同じ集合か返す
    return root(x) == root(y);
  }
  int root(int x) { // xのルートを返す
    return data[x] < 0 ? x : data[x] = root(data[x]);
  }
  int size(int x) { // xの集合のサイズを返す
    return -data[root(x)];
  }
  int numSet() { // 集合の数を返す
    return num;
  }
};
class SumSegTree {
private:
  int _sum(int a, int b, int k, int l, int r) {
    if (r <= a || b <= l)
      return 0; // 交差しない
    if (a <= l && r <= b)
      return dat[k]; // a,l,r,bの順で完全に含まれる
    else {
      int s1 = _sum(a, b, 2 * k + 1, l, (l + r) / 2); // 左の子
      int s2 = _sum(a, b, 2 * k + 2, (l + r) / 2, r); // 右の子
      return s1 + s2;
    }
  }

public:
  int n, height;
  vector<int> dat;

  // 初期化（_nは最大要素数）
  SumSegTree(int _n) {
    n = 1;
    height = 1;
    while (n < _n) {
      n *= 2;
      height++;
    }
    dat = vector<int>(2 * n - 1, 0);
  }

  // 場所i(0-indexed)にxを足す
  void add(int i, int x) {
    i += n - 1; // i番目の葉ノードへ
    dat[i] += x;
    while (i > 0) { // 下から上がっていく
      i = (i - 1) / 2;
      dat[i] += x;
    }
  }

  // 区間[a,b)の総和。ノードk=[l,r)に着目している。
  int sum(int a, int b) { return _sum(a, b, 0, 0, n); }
};
class RmqTree {
private:
  ll _find(ll a, ll b, ll k, ll l, ll r) {
    if (r <= a || b <= l)
      return INF; // 交差しない
    if (a <= l && r <= b)
      return dat[k]; // a,l,r,bの順で完全に含まれる
    else {
      ll s1 = _find(a, b, 2 * k + 1, l, (l + r) / 2); // 左の子
      ll s2 = _find(a, b, 2 * k + 2, (l + r) / 2, r); // 右の子
      return min(s1, s2);
    }
  }

public:
  ll n, height;
  vector<ll> dat;

  // 初期化（_nは最大要素数）
  RmqTree(ll _n) {
    n = 1;
    height = 1;
    while (n < _n) {
      n *= 2;
      height++;
    }
    dat = vector<ll>(2 * n - 1, INF);
  }

  // 場所i(0-indexed)をxにする
  void update(ll i, ll x) {
    i += n - 1; // i番目の葉ノードへ
    dat[i] = x;
    while (i > 0) { // 下から上がっていく
      i = (i - 1) / 2;
      dat[i] = min(dat[i * 2 + 1], dat[i * 2 + 2]);
    }
  }

  // 区間[a,b)の最小値。ノードk=[l,r)に着目している。
  ll find(ll a, ll b) { return _find(a, b, 0, 0, n); }
};

// 約数求める //約数
void divisor(ll n, vector<ll> &ret) {
  for (ll i = 1; i * i <= n; i++) {
    if (n % i == 0) {
      ret.push_back(i);
      if (i * i != n)
        ret.push_back(n / i);
    }
  }
  sort(ret.begin(), ret.end());
}

vector<ll> lis_fast(const vector<ll> &a) { // 最長部分増加列
  const ll n = a.size();
  vector<ll> A(n, INT_MAX);
  vector<ll> id(n);
  for (int i = 0; i < n; ++i) {
    id[i] = distance(A.begin(), lower_bound(A.begin(), A.end(), a[i]));
    A[id[i]] = a[i];
  }
  ll m = *max_element(id.begin(), id.end());
  vector<ll> b(m + 1);
  for (int i = n - 1; i >= 0; --i)
    if (id[i] == m)
      b[m--] = a[i];
  return b;
}

ll ModPow(ll x, ll n) {
  ll res = 1LL;
  while (n > 0) {
    if (n & 1)
      res = res * x % MOD;
    x = x * x % MOD;
    n >>= 1;
  }
  return res;
}
// nCrとか
class Combination {
public:
  Array fact;
  Array inv;
  ll mod;
  ll mod_inv(ll x) {
    ll n = mod - 2LL;
    ll res = 1LL;
    while (n > 0) {
      if (n & 1)
        res = res * x % mod;
      x = x * x % mod;
      n >>= 1;
    }
    return res;
  }
  ll nCr(ll n, ll r) { return ((fact[n] * inv[r] % mod) * inv[n - r]) % mod; }
  ll nPr(ll n, ll r) { return (fact[n] * inv[n - r]) % mod; }
  ll nHr(ll n, ll r) { return nCr(r + n - 1, r); }
  Combination(ll n, ll _mod) {
    mod = _mod;
    fact.resize(n + 1);
    fact[0] = 1;
    REP(i, n) { fact[i + 1] = (fact[i] * (i + 1LL)) % mod; }
    inv.resize(n + 1);
    REP(i, n + 1) { inv[i] = mod_inv(fact[i]); }
  }
};

ll gcd(ll m, ll n) {
  if (n == 0)
    return m;
  return gcd(n, m % n);
} // gcd

ll lcm(ll m, ll n) { return m / gcd(m, n) * n; }

Matrix mIdentity(ll n) {
  Matrix A(n, Array(n));
  for (int i = 0; i < n; ++i)
    A[i][i] = 1;
  return A;
}

Matrix mMul(const Matrix &A, const Matrix &B) {
  Matrix C(A.size(), Array(B[0].size()));
  for (int i = 0; i < C.size(); ++i)
    for (int j = 0; j < C[i].size(); ++j)
      for (int k = 0; k < A[i].size(); ++k)
        (C[i][j] += (A[i][k] % MOD) * (B[k][j] % MOD)) %= MOD;
  return C;
}
// O( n^3 log e )
Matrix mPow(const Matrix &A, ll e) {
  return e == 0       ? mIdentity(A.size())
         : e % 2 == 0 ? mPow(mMul(A, A), e / 2)
                      : mMul(A, mPow(A, e - 1));
}

Matrix used(100100, Array(3, 0));
ll cost[100100];

void dfs(Graph &graph, ll v, ll num) {
  used[v][num % 3] = num;
  if (num % 3 == 0)
    chmin(cost[v], num);
  REP(i, graph[v].size()) {
    if (used[graph[v][i].to][(num + 1) % 3] > num + 1) {
      dfs(graph, graph[v][i].to, num + 1);
    }
  }
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);

  ll n, m;
  cin >> n >> m;
  Graph graph(n);
  REP(i, m) {
    ll u, v;
    cin >> u >> v;
    u--;
    v--;
    add_edge(graph, u, v, 1, false, 0);
  }
  ll s, t;
  cin >> s >> t;
  s--;
  t--;
  REP(i, n) cost[i] = INF;
  REP(i, n) {
    REP(j, 3) { used[i][j] = INF; }
  }
  queue<P> que;
  que.push(make_pair(s, 0));
  used[s][0] = 0;
  ll i = 0;
  while (que.size()) {
    ll v = que.front().first;
    ll num = que.front().second;
    REP(i, graph[v].size()) {
      if (used[graph[v][i].to][(num + 1) % 3] > num + 1) {
        if (graph[v][i].to == t && (num + 1) % 3 == 0) {
          cout << (num + 1) / 3;
          return 0;
        }
        que.push(make_pair(graph[v][i].to, num + 1));
        used[graph[v][i].to][(num + 1) % 3] = num + 1;
      }
    }
    que.pop();
    i++;
    if (i >= 90000000) {
      cout << -1;
      return 0;
    }
  }

  cout << -1;

  return 0;
}