//#define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
#define rep(i, n) for(int i=0; i<n; ++i)
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
using namespace std;
using ll = int64_t;
using ld = long double;
using P = pair<int, int>;
using vs = vector<string>;
using vi = vector<int>;
using vvi = vector<vi>;
template<class T> using PQ = priority_queue<T>;
template<class T> using PQG = priority_queue<T, vector<T>, greater<T> >;
const int INF = 0xccccccc;
const ll LINF = 0xcccccccccccccccLL;
template<typename T1, typename T2>
inline bool chmax(T1 &a, T2 b) {return a < b && (a = b, true);}
template<typename T1, typename T2>
inline bool chmin(T1 &a, T2 b) {return a > b && (a = b, true);}
template<typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) { return is >> p.first >> p.second;}
template<typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) { return os << p.first << ' ' << p.second;}



//head



int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	int n, m, a, b;
	cin >> n >> m >> a >> b;
	vvi G(n+1);
	rep(i, m) {
		int l, r;
		cin >> l >> r;
		G[l-1].emplace_back(r);
		G[r].emplace_back(l-1);
	}
	vi dist(n+1, INF);
	queue<P> q;
	bitset<100100> bits(0);
	rep(i, a) {
		bits.set(i);
		q.emplace(i, 0);
	}
	while(!q.empty()) {
		auto [now, d] = q.front();
		q.pop();
		for(int ne:G[now]) {
			if(bits.test(ne)) continue;
			if(ne >= b) {
				cout << d+1 << endl;
				return 0;
			}
			q.emplace(ne, d+1);
			bits.set(ne);
		}
	}
	cout << "-1\n";
}