#include <algorithm>
#include <cassert>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <iostream>
#include <numeric>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <functional>
#include <iomanip>
using namespace std;
using ll = long long;

class range {private: struct I{int x;int operator*(){return x;}bool operator!=(I& lhs){return x<lhs.x;}void operator++(){++x;}};I i,n;
public:range(int n_):i({0}),n({n_}){}range(int i_,int n_):i({i_}),n({n_}){}I& begin(){return i;}I& end(){return n;}};

template<typename T, typename U> ostream& operator<<(ostream& os, const pair<T, U>& p){ return os << "{" << p.first << ", " << p.second << "}"; }
template<typename T> ostream& operator<<(ostream& os, const vector<T>& obj) { os << "{"; for (const auto& e : obj) os << e << ", "; return os << "}"; }
template<typename T> ostream& operator<<(ostream& os, const set<T>& obj) { os << "set{"; for (const auto& e : obj) os << e << ", "; return os << "}"; }
template<typename T, typename U> ostream& operator<<(ostream& os, const map<T, U>& obj) { os << "map{"; for (const auto& e : obj) os << e << ", "; return os << "}"; }

#ifdef ONLINE_JUDGE
#define dump(expr) ;
#else
#define dump(expr) { cerr << "\033[33m#L" << __LINE__ << ": " << expr << "\033[39m" << endl; }
#endif

using EdgeCost = ll;
struct E {
  int fr, to, eid;
  EdgeCost cost;
  E(int fr_, int to_, int eid_, EdgeCost cost_) : fr(fr_), to(to_), eid(eid_), cost(cost_) {}
  friend ostream &operator<<(ostream &os, const E &e) { os << "(" << e.fr << " -> " << e.to << ")"; return os; }
};
using V = vector<E>;
struct G {
  int n, e_count;
  vector<V> adj;
  vector<V> rev;
  G(int n_) : n(n_), adj(n), rev(n) {}

  void add_directed_edge(int fr, int to, EdgeCost cost = (EdgeCost)1) {
    int eid = e_count;
    adj[fr].push_back(E(fr, to, eid, cost));
    rev[to].push_back(E(fr, to, eid, cost));
    e_count++;
  }

  void add_undirected_edge(int fr, int to, EdgeCost cost = (EdgeCost)1) {
    int eid = e_count;
    adj[fr].push_back(E(fr, to, eid, cost));
    rev[to].push_back(E(fr, to, eid, cost));
    adj[to].push_back(E(to, fr, eid, cost));
    rev[fr].push_back(E(to, fr, eid, cost));
    e_count++;
  }
};

EdgeCost shortest_path(G& g, int src, int dst) {
  vector<char> vis(g.n, 0);
  priority_queue<pair<EdgeCost, int>> q;
  q.push(make_pair((EdgeCost)0, src));

  while (!q.empty()) {
    EdgeCost curr_cost = -q.top().first;
    int idx = q.top().second;
    q.pop();
    if (vis[idx])
      continue;
    vis[idx] = 1;

    if (idx == dst) {
      return curr_cost;
    }

    for (auto it : g.adj[idx]) {
      if (vis[it.to])
        continue;
      q.push({-(curr_cost + it.cost), it.to});
    }
  }

  return (EdgeCost)-1; // impossible
}

EdgeCost shortest_path_1244(G& g, int src, int dst) {
  vector<char> vis(g.n, 0);
  priority_queue<pair<EdgeCost, int>> q;
  for (int src0 : range(src + 1)) q.push(make_pair((EdgeCost)0, src0));

  while (!q.empty()) {
    EdgeCost curr_cost = -q.top().first;
    int idx = q.top().second;
    q.pop();
    if (vis[idx])
      continue;
    vis[idx] = 1;

    if (idx >= dst) {
      return curr_cost;
    }

    for (auto it : g.adj[idx]) {
      if (vis[it.to])
        continue;
      q.push({-(curr_cost + it.cost), it.to});
    }
  }

  return (EdgeCost)-1; // impossible
}

namespace solver {

int n, m, a, b;
vector<int> vl, vr;

void read() {
  cin >> n >> m >> a >> b;
  vl.resize(m);
  vr.resize(m);
  for (int i : range(m)) cin >> vl[i] >> vr[i];
}

ll run() {
  G g(n + 2);
  for (int i : range(m)) {
    g.add_undirected_edge(vl[i], vr[i] + 1, 1);
  }
  dump(g.adj);

  ll ans = shortest_path_1244(g, a, b + 1);
  return ans;
}

}  // namespace

int main(int argc, char** argv) {
  cerr << fixed << setprecision(12);
  cout << fixed << setprecision(12);
  int testcase = 1;
  if (argc > 1) testcase = atoi(argv[1]);
  while (testcase--) {
    solver::read();
  }
  cout << solver::run() << endl;
}