const fi='';//'STNODE.inp';
      fo='';//'STNODE.out';
type ptr=^node;
     node=record
       v : longint;
       next:ptr;
       end;
var  f1,f2 : text;
     tr,thu,q : array[0..10000] of longint;
     f,l,i,dem,kq,n,m,s,t,v,u : longint;
     k : array[0..10000] of ptr;
     inq :array[0..10000] of boolean;
     p : ptr;
procedure tinh;
  var w : longint; p:ptr;
  BEGIN
  w:=t;
  while w<>s do
    BEGIN
    w:=tr[w];
    if w<>s then
      BEGIN
    inc(dem);
 
    thu[dem]:=w;
    end;
    end;
  end;
procedure bfs(u : longint);
  var p:ptr;
  BEGIN
   f:=1;
 
   l:=1;
   fillchar(inq,sizeof(inq),false);
   inq[u]:=true;
//   inq[k]:=true;
   q[f]:=u;
   repeat
   v:=q[f]; inc(f);
   p:=k[v];
   while p<> nil do
      if inq[p^.v]=false then
         BEGIN
         inc(l);
         q[f]:=p^.v;
         tr[p^.v]:=v;
         inq[p^.v]:=true;
         if p^.v=t then
         BEGIN
            tinh;
            exit;
         end;
         p:=p^.next;
         end
         else p:=p^.next;
   until f>l;
   end;
function bfs2(u,o : longint): boolean;
  var p:ptr; v: longint;
  BEGIN
  fillchar(q,sizeof(q),0);
  bfs2:=true;
  f:=1;
  l:=1;
  fillchar(inq,sizeof(inq),false);
  inq[u]:=true;
  inq[o]:=true;
  q[f]:=u;
  repeat
    v:=q[f];
    inc(f);
    p:=k[v];
    while p<> nil do
        if inq[p^.v]=false then
           BEGIN
           inc(l);
           q[f]:=p^.v;
           inq[p^.v]:=true;
           if p^.v= t then
           BEGIN
           bfs2:=false;
           exit;
           end;
           p:=p^.next;
           end
           else p:=p^.next;
  until f>l;
  end;
BEGIN
assign(f1,fi);
reset(f1);
assign(f2,fo);
rewrite(f2);
read(f1,n,m,s,t);
for i:=1 to m do
   BEGIN
   read(f1,u,v);
   new(p);
   p^.v:=v;
   p^.next:=k[u];
   k[u]:=p;
   end;
//for i:=1 to n do k[i]:=nil;
bfs(s);
kq:=0;
if dem=0 then
  BEGIN
  write(f2,0);
  halt;
  end;
for i:=1 to dem do if bfs2(s,thu[i]) then inc(kq);
write(f2,kq);
close(f1);
close(f2);
end.