#include <cassert>
#include <cmath>
#include <algorithm>
#include <iostream>
#include <iomanip>
#include <limits.h>
#include <map>
#include <queue>
#include <set>
#include <string.h>
#include <vector>

using namespace std;
typedef long long ll;

struct Node {
  int v;
  int dist;

  Node(int v = -1, int dist = -1) {
    this->v = v;
    this->dist = dist;
  }

  bool operator>(const Node &n) const {
    return dist > n.dist;
  }
};

const int MAX_N = 100'010;
vector<int> E[MAX_N];

int main() {
  int N, M, A, B;
  cin >> N >> M >> A >> B;

  for (int i = 0; i < M; ++i) {
    int l, r;
    cin >> l >> r;
    ++r;

    l = max(l, A);
    r = min(r, B + 1);

    E[l].push_back(r);
    E[r].push_back(l);
  }

  priority_queue <Node, vector<Node>, greater<Node>> pque;
  pque.push(Node(A, 0));
  bool visited[MAX_N];
  memset(visited, false, sizeof(visited));

  while (!pque.empty()) {
    Node node = pque.top();
    pque.pop();

    if (visited[node.v]) continue;
    visited[node.v] = true;

    if (node.v == B + 1) {
      cout << node.dist << endl;
      return 0;
    }

    for (int u : E[node.v]) {
      pque.push(Node(u, node.dist + 1));
    }
  }

  cout << -1 << endl;

  return 0;
}