Program city;

Type
sc1=array [1..2500,1..2] of integer;

Var
sc:sc1;
b,i,j,min,p:integer;
n,m,s,e:integer;
f1,f2:text;

Begin
assign(f1,'city.in');
assign(f2,'city.out');
reset(f1);
rewrite(f2);
b:=1;
readln(f1,n,m,s,e);
for i:=1 to m do
 for j:=1 to 2 do
  read(f1,sc[i,j]);
for i:=1 to m do begin
 for j:=1 to m do begin
  if sc[j,1]=s then begin
   s:=sc[j,2];
   b:=b+1;
   if s=e then write(f2,b);
  end;
 end;
end;
close(f1);
close(f2);
End.