#include <iostream>
#include <cstdio>
#include <cmath>
#include <ctime>
#include <cstdlib>
#include <cassert>
#include <vector>
#include <list>
#include <stack>
#include <queue>
#include <deque>
#include <map>
#include <set>
#include <bitset>
#include <string>
#include <algorithm>
#include <utility>
#define llint long long
#define inf 1e18
#define rep(x, s, t) for(llint (x) = (s); (x) < (t); (x)++)
#define Rep(x, s, t) for(llint (x) = (s); (x) <= (t); (x)++)
#define chmin(x, y) (x) = min((x), (y))
#define chmax(x, y) (x) = max((x), (y))

using namespace std;
typedef pair<llint, llint> P;

llint n, m, a, b;
vector<llint> G[100005];
llint dist[100005];

void bfs(vector<llint> G[], int S, llint dist[])
{
	queue<int> Q;
	Q.push(S);
	dist[S] = 0;
	
	while(Q.size()){
		int v = Q.front();
		Q.pop();
		for(int i = 0; i < G[v].size(); i++){
			if(dist[G[v][i]] < inf/2) continue;
			Q.push(G[v][i]);
			dist[G[v][i]] = dist[v] + 1;
		}
	}
}

int main(void)
{
	ios::sync_with_stdio(0);
	cin.tie(0);
	
	cin >> n >> m >> a >> b;
	llint l, r;
	for(int i = 1; i <= m; i++){
		cin >> l >> r;
		G[l-1].push_back(r);
		G[r].push_back(l-1);
	}
	
	llint S = n+1, T = n+2;
	for(int i = 0; i < a; i++) G[S].push_back(i);
	for(int i = b; i <= n; i++) G[i].push_back(T);
	
	for(int i = 0; i <= T; i++) dist[i] = inf;
	bfs(G, S, dist);
	
	llint ans = dist[T]-dist[S]-2;
	if(ans > inf/2) cout << -1 << endl;
	else cout << ans << endl;
	
	return 0;
}