#include <bits/stdc++.h>
using namespace std;
 
#define fi first
#define se second
#define pb push_back
using vi = vector <int>;
using ll = long long;
using pii = pair <int, int>;
//~ const ll mod = 998244353;
const ll mod = 1e9 + 7;
ll qpow(ll a, ll b, ll m = mod) { ll r = 1, t = a;
	for(; b; b /= 2) { if(b & 1) r = r * t % m; t = t * t % m; } return r; }

const int N = 1e5 + 11;
vi e[N];

int dis[N];
const int inf = 1e9;

int main() {
	ios :: sync_with_stdio(false);
	
	int n, m; cin >> n >> m;
	int a, b; cin >> a >> b;
	for(int i = 0; i < m; i ++) {
		int l, r; cin >> l >> r;
		e[l - 1].pb(r);
		e[r].pb(l - 1);
	}
	
	int S = n + 1, T = n + 2;
	for(int i = 0; i < a; i ++)
		e[S].pb(i);
	for(int i = b; i <= n; i ++)
		e[i].pb(T);
	
	fill(dis, dis + T + 1, inf);
	queue <int> q; q.push(S);
	dis[S] = 0;
	
	while(!q.empty()) {
		int x = q.front(); q.pop();
		for(int i : e[x])
			if(dis[i] == inf) {
				dis[i] = dis[x] + 1;
				q.push(i);
			}
	}
	
	int ans = (dis[T] == inf ? -1 : dis[T] - 2);
	cout << ans << '\n';
}