program ex(f1,f2);
 type
  zn=array [1..50] of integer;
  ha=array [1..50,1..50] of boolean;
 var
  a:zn; flag:ha; f1,f2:text; i,j,k,n,m,first,last:integer;
  flag2:boolean; answer,head,tail:integer;
 begin
  assign(f1,'city.in'); reset(f1);
  assign(f2,'city.out'); rewrite(f2);
  readln(f1,n,m,first,last);
  for i:=1 to m do
   begin
    readln(f1,j,k);
    flag[j,k]:=true;
   end;
  flag2:=true;
  head:=1; tail:=1;
  answer:=1; a[head]:=first;
  while flag2 do
   begin
    inc(answer);
    for j:=head to tail do
    for i:=1 to n do
     if flag[a[j],i]=true then
       begin
        inc(tail); a[tail]:=i;
        if a[tail]=last then
         begin  writeln(f2,answer); flag2:=false end;
       end;
    inc(head);
   end;
  close(f1);
  close(f2);
 end.