// #pragma GCC optimize("Ofast")
// #pragma GCC target("avx2,bmi,bmi2,lzcnt")
// #define NDEBUG

#include <bits/extc++.h>
#include <x86intrin.h>

struct rep {
  struct iter {
    int i;
    constexpr void operator++() { ++i; }
    constexpr int operator*() const { return i; }
    friend constexpr bool operator!=(iter a, iter b) { return *a != *b; }
  };
  const int l, r;
  constexpr rep(int _l, int _r) : l(std::min(_l, _r)), r(_r) {}
  constexpr rep(int n) : rep(0, n) {}
  constexpr iter begin() const { return {l}; }
  constexpr iter end() const { return {r}; }
};
struct per {
  struct iter {
    int i;
    constexpr void operator++() { --i; }
    constexpr int operator*() const { return i; }
    friend constexpr bool operator!=(iter a, iter b) { return *a != *b; }
  };
  const int l, r;
  constexpr per(int _l, int _r) : l(std::min(_l, _r)), r(_r) {}
  constexpr per(int n) : per(0, n) {}
  constexpr iter begin() const { return {r - 1}; }
  constexpr iter end() const { return {l - 1}; }
};
template <class T, class U>
constexpr bool chmin(T& a, U&& b) {
  return b < a ? a = std::forward<U>(b), true : false;
}
template <class T, class U>
constexpr bool chmax(T& a, U&& b) {
  return a < b ? a = std::forward<U>(b), true : false;
}

struct graph {
  struct edge {
    int src, dst, cost;

    int operator-(int v) const {
      assert(v == src or v == dst);
      return src ^ dst ^ v;
    }
  };

  int n, m = 0;
  std::vector<edge> edges;
  std::vector<std::vector<std::pair<int, int>>> adj;
  std::function<bool(int)> ignore = [](int) { return false; };

  graph() : n(0) {}
  explicit graph(int _n) : n(_n), adj(n) {}

  int add(const edge& e, bool directed) {
    assert(0 <= e.src), assert(e.src < n);
    assert(0 <= e.dst), assert(e.dst < n);
    edges.push_back(e);
    adj[e.src].emplace_back(m, e.dst);
    if (not directed) adj[e.dst].emplace_back(m, e.src);
    return m++;
  }
};

using namespace std;

template <class T>
pair<vector<T>, vector<int>> dijkstra(const graph& g, int s) {
  vector d(g.n, numeric_limits<T>::max()), pe(g.n, -1);
  priority_queue<pair<T, int>, vector<pair<T, int>>, greater<>> pq;
  pq.emplace(d[s] = 0, s);
  while (not empty(pq)) {
    auto [dv, v] = pq.top();
    pq.pop();
    if (dv > d[v]) continue;
    for (auto [id, u] : g.adj[v])
      if (dv + g.edges[id].cost < d[u]) {
        d[u] = dv + g.edges[id].cost;
        pq.emplace(d[u], u);
        pe[u] = id;
      }
  }
  return {d, pe};
}

int main() {
  using namespace std;
  cin.tie(nullptr)->sync_with_stdio(false);
  int n, m, a, b;
  cin >> n >> m >> a >> b;
  --a;
  int s = n + 1, t = s + 1;
  graph g(t + 1);
  while (m--) {
    int l, r;
    cin >> l >> r;
    --l;
    g.add({l, r, 1}, false);
  }
  for (int v : rep(a + 1)) g.add({s, v}, true);
  for (int v : rep(b, n + 1)) g.add({v, t}, true);
  auto d = dijkstra<int>(g, s).first;
  if (d[t] <= n) {
    cout << d[t] << '\n';
  } else {
    cout << "-1\n";
  }
}