package chelper;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;

import io.InputReader;
import io.OutputWriter;

public class L {
  int[] bfs(List<List<Integer>> edges, int from) {
    int n = edges.size();
    int[] res = new int[n];
    Arrays.fill(res, n + 100);
    res[from] = 0;

    boolean[] visited = new boolean[n];
    visited[from] = true;

    Queue<Integer> queue = new LinkedList<>();
    queue.add(from);

    while (!queue.isEmpty()) {
      int a = queue.poll();
      for (int b : edges.get(a)) {
        if (!visited[b]) {
          res[b] = res[a] + 1;
          visited[b] = true;
          queue.add(b);
        }
      }
    }

    return res;
  }

  public void solve(int testNumber, InputReader in, OutputWriter out) {
    int n = in.nextInt() + 1;
    int m = in.nextInt();

    int a = in.nextInt();
    int b = in.nextInt();

    List<List<Integer>> edges1 = new ArrayList<>();
    List<List<Integer>> edges2 = new ArrayList<>();

    for (int i = 0; i < n; i++) {
      edges1.add(new ArrayList<>());
      edges2.add(new ArrayList<>());
    }

    for (int i = 0; i < m; i++) {
      int v1 = in.nextInt();
      int v2 = in.nextInt();
      edges1.get(v1).add(v2);
      edges2.get(v2).add(v1);
    }

    int[] toPivot = bfs(edges1, 0);
    int[] fromA = bfs(edges2, a);
    int[] fromB = bfs(edges2, b);

    int res = n + 100;

    for (int i = 0; i < n; i++) {
      res = Math.min(res, toPivot[i] + fromA[i] + fromB[i]);
    }

    out.println(res);
  }
}