import java.io.*; import java.util.*; import java.util.stream.*; public class Main { public static void main(String[] args) throws Exception { Scanner sc = new Scanner(); int n = sc.nextInt(); int m = sc.nextInt(); int a = sc.nextInt() - 1; int b = sc.nextInt() - 1; ArrayList> graph = new ArrayList<>(); for (int i = 0; i < n + 1; i++) { graph.add(new ArrayList<>()); } for (int i = 0; i < m; i++) { int x = sc.nextInt() - 1; int y = sc.nextInt(); graph.get(x).add(y); graph.get(y).add(x); } PriorityQueue queue = new PriorityQueue<>(); for (int i = 0; i <= a; i++) { queue.add(new Path(i, 0)); } int[] costs = new int[n + 1]; Arrays.fill(costs, Integer.MAX_VALUE); while (queue.size() > 0) { Path p = queue.poll(); if (costs[p.idx] <= p.value) { continue; } costs[p.idx] = p.value; for (int x : graph.get(p.idx)) { queue.add(new Path(x, p.value + 1)); } } int ans = Integer.MAX_VALUE; for (int i = b + 1; i <= n; i++) { ans = Math.min(ans, costs[i]); } if (ans == Integer.MAX_VALUE) { System.out.println(-1); } else { System.out.println(ans); } } static class Path implements Comparable { int idx; int value; public Path(int idx, int value) { this.idx = idx; this.value = value; } public int compareTo(Path another) { return value - another.value; } } } class Utilities { static String arrayToLineString(Object[] arr) { return Arrays.stream(arr).map(x -> x.toString()).collect(Collectors.joining("\n")); } static String arrayToLineString(int[] arr) { return String.join("\n", Arrays.stream(arr).mapToObj(String::valueOf).toArray(String[]::new)); } } class Scanner { BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); StringTokenizer st = new StringTokenizer(""); StringBuilder sb = new StringBuilder(); public Scanner() throws Exception { } public int nextInt() throws Exception { return Integer.parseInt(next()); } public long nextLong() throws Exception { return Long.parseLong(next()); } public double nextDouble() throws Exception { return Double.parseDouble(next()); } public int[] nextIntArray() throws Exception { return Stream.of(br.readLine().split(" ")).mapToInt(Integer::parseInt).toArray(); } public String next() throws Exception { while (!st.hasMoreTokens()) { st = new StringTokenizer(br.readLine()); } return st.nextToken(); } }