BAEKJOON 14496

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.PriorityQueue;
import java.util.StringTokenizer;

public class Main {

	static int a,b,N,M, ans = Integer.MAX_VALUE;
	static boolean[] visit;
	static ArrayList<Integer>[] list;
	static PriorityQueue<int[]> pq = new PriorityQueue<>((a,b) -> a[1] - b[1]);
	public static void main(String[] args) throws Exception {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		StringTokenizer st = new StringTokenizer(br.readLine());
		a = Integer.parseInt(st.nextToken());
		b = Integer.parseInt(st.nextToken());
		
		st = new StringTokenizer(br.readLine());
		N = Integer.parseInt(st.nextToken());
		M = Integer.parseInt(st.nextToken());
		
		visit = new boolean[N+1];
		list = new ArrayList[N+1];
		for (int i = 0; i < N+1; i++) {
			list[i] = new ArrayList<>();
		}
		
		for (int i = 0; i < M; i++) {
			st = new StringTokenizer(br.readLine());
			int s = Integer.parseInt(st.nextToken());
			int e = Integer.parseInt(st.nextToken());
			list[s].add(e);
			list[e].add(s);
		}
		
		pq.add(new int[] {a,0});
		visit[a] = true;
		
		while(!pq.isEmpty()) {
			int[] n = pq.poll();
			if(n[0] == b) {
				ans = Math.min(ans, n[1]);
				continue;
			}
			for (int i = 0; i < list[n[0]].size(); i++) {
				int next = list[n[0]].get(i);
				if(!visit[next]) {
					pq.add(new int[] {next,n[1]+1});
					visit[next] = true;
				}
			}
		}
		if (ans == Integer.MAX_VALUE)
			System.out.println(-1);
		else
			System.out.println(ans);
	}
}