import java.util.ArrayList;
import java.util.LinkedList;
// ADDITIONAL IMPORTS ARE NOT ALLOWED

class Main {
  public static void main(String[] args) {
    // Uncomment the following two lines if you want to read from a file
    In.open("public/small.in");
    Out.compareTo("public/small.out");

    int n = In.readInt(); //number of vertices
    int edges = In.readInt();
    int x = In.readInt();
    int y = In.readInt();
    ArrayList<ArrayList<Integer>> G = new ArrayList<ArrayList<Integer>>(n);
    ArrayList<ArrayList<Integer>> G_r = new ArrayList<ArrayList<Integer>>(n);

    for (int i = 0; i < n; i++) {
      G.add(new ArrayList<Integer>());
      G_r.add(new ArrayList<Integer>());
    }

    for (int i = 0; i < edges; i++) {
      int src = In.readInt();
      int trg = In.readInt();
      G.get(src).add(trg);
      G_r.get(trg).add(src);
    }

    Out.println(countInBetween(n, G,G_r, x, y));

    // Uncomment this line if you want to read from a file
    In.close();
  }

  public static int countInBetween(int n, ArrayList<ArrayList<Integer>> G,ArrayList<ArrayList<Integer>> G_r, int x, int y) {
    boolean[] G_mark = new boolean[n];
    boolean[] G_mark_r = new boolean[n];
    ArrayList<Integer> list_forward = new ArrayList<Integer>();
    ArrayList<Integer> list_backward = new ArrayList<Integer>();
    BFS(x,G,G_mark,list_forward);
    BFS(y,G_r,G_mark_r,list_backward);

    list_forward.retainAll(list_backward);
    return list_forward.size();

  }

  public static ArrayList<Integer> dfs(ArrayList<ArrayList<Integer>> G, boolean[] G_mark, int node, ArrayList<Integer> list){
    G_mark[node] = true;
    if(!list.contains(node)){
      list.add(node);
    }
    for(int next: G.get(node)){
      if(!G_mark[next]){
          list.addAll(dfs(G,G_mark,next,new ArrayList<Integer>()));
      }
    }

    return list;
  }

  public static void BFS(int x,ArrayList<ArrayList<Integer>> G, boolean[] G_mark, ArrayList<Integer> list){
    LinkedList<Integer> queue = new LinkedList<Integer>();
    G_mark[x] = true;
    queue.add(x);
    while(queue.size()!=0){
      x = queue.poll();
      list.add(x);
      for(int next: G.get(x)){
        if(!G_mark[next]){
          G_mark[next] = true;
          queue.add(next);
        }
      }
    }
  }


}