program citys;

var road:array[1..50,1..50]of boolean;
    city:array[1..50]of boolean;
    a,b,n,m,s,e,i,min:integer;
    f:text;

procedure search(a,far:integer);
 var i,j:integer;
 begin
  if (a=e)and(far<min) then
   min:=far;

  for i:=1 to n do
   if (road[a,i])and(not(city[i])) then
   begin
    city[i]:=true;
    search(i,far+1);
    city[i]:=false;
   end;
 end;

begin
 assign(f,'city.in');
 reset(f);
 readln(f,n,m,s,e);
 min:=maxint;

 for i:=1 to m do
 begin
  readln(f,a,b);
  road[a,b]:=true;
 end;
 close(f);

 search(s,1);

 assign(f,'city.out');
 rewrite(f);
 writeln(f,min);
 close(f);
end.