#include <bits/stdc++.h>
#include <atcoder/modint>
using namespace atcoder;
using namespace std;
using ll = long long;
using mint = modint998244353;
int main() {
	int N, M, A, B;cin >> N >> M >> A >> B;
	vector<vector<int>> G(N + 4);
	for (int i = 0;i < M;i++) {
		int l, r;cin >> l >> r;
		G[l].push_back(r + 1);
		G[r + 1].push_back(l);
	}
	for (int i = 1;i <= A;i++) G[N + 2].push_back(i);
	for (int i = B + 1;i <= N + 1;i++) G[i].push_back(N + 3);
	queue<int> que;que.push(N + 2);
	vector<int> dist(N + 4, -1);
	dist[N + 2] = 0;
	while (!que.empty()) {
		int v = que.front();que.pop();
		for (auto nv : G[v]) {
			if (dist[nv] == -1) {
				dist[nv] = dist[v] + 1;
				que.push(nv);
			}
		}
	}
	cout << max(-1, dist[N + 3]  - 2) << endl;
}