const fi='';
        fo='';
var a:array[1..1000000]of longint;
       tham:array[1..10000]of boolean;
       q,cha,b,head:array[1..10000]of longint;
       l, n,m,s,t,max:longint; co:boolean;
procedure nhap;
var last:array[1..10000]of longint;
        u,v,i:longint;
        f:text;
begin
assign(f,fi);
reset(f);
readln(f,n,m,s,t);
head[1]:=1;
while not eof(f) do
begin
readln(f,u,v);
head[u+1]:=head[u+1]+1;
end;
for i:=1 to n do
head[i+1]:=head[i+1]+head[i];
close(f);
reset(f);
readln(f);
last:=head;
while not eof(F) do
begin
readln(f,u,v);
a[last[u]]:=v;
inc(last[u]);
end;
close(f);
end;
 
procedure bfs(s:longint);
var u,v,dau,cuoi:longint;
begin
dau:=1;
cuoi:=1;
q[dau]:=s;
repeat
u:=q[dau];
for v:=head[u] to head[u+1]-1 do
if (not tham[a[v]]) then
                                begin
                                cuoi:=cuoi+1;
                                q[cuoi]:=a[v];
                                cha[a[v]]:=u;
                                if v=t then begin
 
                                co:=true;
                                exit;
                                end;
 
                                end;
   dau:=dau+1;
until dau>cuoi;
end;
 
 
procedure timduong;
var i,k:longint;
begin
k:=cha[t];
i:=0;
repeat
inc(i);
b[i]:=k;
k:=cha[k];
until k=s;
l:=i;
end;
 
procedure xuli;
var i:longint;
begin
max:=0;
bfs(s);
timduong;
co:=false;
for i:=1 to l do
                begin
                fillchar(q,sizeof(q),0);
                fillchar(tham,sizeof(tham),false);
                tham[b[i]]:=true;
                bfs(s);
                if not co then max:=max+1
                else
                co:=false;
                end;
end;
 
procedure xuat;
var f:text;
begin
assign(f,fo);
rewrite(f);
writeln(f,max);
close(f)
end;
 
begin
nhap;
xuli;
xuat;
end.