program city;
type
shuzu=array[1..100,1..100] of boolean;
shuzu1=array[1..100,1..100] of integer;
var
s:shuzu;
d:shuzu1;
i,j,x,y,m,n,b,e,c,temp,now:integer;
f1,f2:text;
procedure try(now:integer);
 var
 i,j:integer;
 begin
     if now=e then writeln(f2,c)
            else
            begin
              for i:=1 to n do
                  if s[now,i]=true then begin
                               s[now,i]:=false;
                               try(i);
                               c:=c+1;
                               s[now,i]:=true;
                               end;
                               end;
                               end;
begin
assign(f1,'city.in');reset(f1);
assign(f2,'city.out');rewrite(f2);
readln(f1,n,m,b,e);
fillchar(s,sizeof(s),0);
for i:=1 to m do
begin
readln(f1,x,y);
s[x,y]:=true;
end;
temp:=0;c:=0;
for i:=1 to n do
begin
  if s[j,i]=true then inc(temp);
       d[j,temp]:=i;
       end;
  for i:=1 to temp do
  begin
  now:=b;
      inc(c);
      try(d[1,i]);
     end;
   close(f2);
   end.