#include "bits/stdc++.h"
using namespace std;
#include "atcoder/all"
using namespace atcoder;
#define int long long
#define REP(i, n) for (int i = 0; i < (int)n; ++i)
#define RREP(i, n) for (int i = (int)n - 1; i >= 0; --i)
#define FOR(i, s, n) for (int i = s; i < (int)n; ++i)
#define RFOR(i, s, n) for (int i = (int)n - 1; i >= s; --i)
#define ALL(a) a.begin(), a.end()
#define IN(a, x, b) (a <= x && x < b)
template<class T>istream&operator >>(istream&is,vector<T>&vec){for(T&x:vec)is>>x;return is;}
template<class T>inline void out(T t){cout << t << "\n";}
template<class T,class... Ts>inline void out(T t,Ts... ts){cout << t << " ";out(ts...);}
template<class T>inline bool CHMIN(T&a,T b){if(a > b){a = b;return true;}return false;}
template<class T>inline bool CHMAX(T&a,T b){if(a < b){a = b;return true;}return false;}
constexpr int INF = 1e18;

signed main(){
	int N, M, A, B;
	cin >> N >> M >> A >> B;
	vector<int>l(M), r(M);
	vector<vector<int>>g(N + 1);
	REP(i, M) {
		cin >> l[i] >> r[i];
		--l[i]; --r[i];
		g[l[i]].emplace_back(r[i] + 1);
		g[r[i] + 1].emplace_back(l[i]);
	}
	vector<int>dist(N + 1, INF);
	queue<int>que;
	REP(i, A) {
		dist[i] = 0;
		que.push(i);
	}
	while(que.size()) {
		auto now = que.front();
		que.pop();
		for(auto e: g[now]) {
			if(CHMIN(dist[e], dist[now] + 1)) {
				que.push(e);
			}
		}
	}
	int ans = INF;
	FOR(i, B, N + 1) CHMIN(ans, dist[i]);
	out((ans == INF) ? -1 : ans);
}