#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
#define FOR(i,m,n) for(int i=(m);i<(n);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
using ll = long long;
constexpr int INF = 0x3f3f3f3f;
constexpr ll LINF = 0x3f3f3f3f3f3f3f3fLL;
constexpr double EPS = 1e-8;
constexpr int MOD = 1000000007;
// constexpr int MOD = 998244353;
constexpr int dy[] = {1, 0, -1, 0}, dx[] = {0, -1, 0, 1};
constexpr int dy8[] = {1, 1, 0, -1, -1, -1, 0, 1}, dx8[] = {0, -1, -1, -1, 0, 1, 1, 1};
template <typename T, typename U> inline bool chmax(T &a, U b) { return a < b ? (a = b, true) : false; }
template <typename T, typename U> inline bool chmin(T &a, U b) { return a > b ? (a = b, true) : false; }
struct IOSetup {
  IOSetup() {
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);
    cout << fixed << setprecision(20);
  }
} iosetup;

int main() {
  int n, m, a, b; cin >> n >> m >> a >> b;
  vector<vector<int>> graph(n + 1);
  while (m--) {
    int l, r; cin >> l >> r;
    graph[l - 1].emplace_back(r);
    graph[r].emplace_back(l - 1);
  }
  vector<int> dist(n + 1, INF);
  queue<int> que;
  REP(i, a) {
    dist[i] = 0;
    que.emplace(i);
  }
  while (!que.empty()) {
    int ver = que.front(); que.pop();
    for (int e : graph[ver]) {
      if (dist[e] == INF) {
        dist[e] = dist[ver] + 1;
        que.emplace(e);
      }
    }
  }
  int ans = INF;
  FOR(i, b, n + 1) chmin(ans, dist[i]);
  cout << (ans == INF ? -1 : ans) << '\n';
  return 0;
}