noip2014 寻找道路

const   maxn=10010;
    maxm=200010;
var nextt,too:array[0..2*maxm] of longint;
    x,y:array[0..maxm] of longint;
    head,q,step:array[0..maxn] of longint;
    vis:array[0..maxn] of boolean;
    n,m,s,t,i,tot:longint;

procedure add(a,b:longint);
begin
    inc(tot);
    nextt[tot]:=head[a];
    too[tot]:=b;
    head[a]:=tot;
end;
function f(u:longint):boolean;
var i:longint;
begin
    i:=head[u];
    while i>0 do begin
        if not vis[too[i]] then exit(false);
        i:=nextt[i];
    end;
    exit(true);
end;
function bfs2:boolean;
var l,r,u,i:longint;
begin
    q[0]:=s;
    step[s]:=0;
    l:=0;
    r:=1;
    while ldo begin
        u:=q[l];
        inc(l);
        if not f(u) then continue;
        i:=head[u];
        while i>0 do begin
            if step[too[i]]=-1 then begin
                step[too[i]]:=step[u]+1;
                q[r]:=too[i];
                inc(r);
                if too[i]=t then begin
                    writeln(step[too[i]]);
                    exit(true);
                end; 
            end;
            i:=nextt[i];
        end;
    end;
    exit(false);
end;
procedure bfs1;
var l,r,u,i:longint;
begin
    q[0]:=t;
    vis[t]:=true;
    l:=0;
    r:=1;
    while ldo begin
        u:=q[l];
        inc(l);
        i:=head[u];
        while i>0 do begin 
            if not vis[too[i]] then begin 
                vis[too[i]]:=true; 
                q[r]:=too[i];
                inc(r);
            end;
            i:=nextt[i];
        end;
    end;
end;
begin
    tot:=0;
    fillchar(vis,sizeof(vis),false);
    fillchar(nextt,sizeof(nextt),0);
    fillchar(head,sizeof(head),0);
    fillchar(too,sizeof(too),0);
    readln(n,m);
    for i:=1 to m do begin
        readln(x[i],y[i]);
        add(y[i],x[i]);
    end;
    readln(s,t);
    bfs1;
    fillchar(nextt,sizeof(nextt),0);
    fillchar(head,sizeof(head),0);
    fillchar(too,sizeof(too),0);
    for i:=0 to maxn do step[i]:=-1;
    fillchar(q,sizeof(q),0);
    tot:=0;
    for i:=1 to m do add(x[i],y[i]);
    if not vis[s] then writeln(-1)
        else if not bfs2 then writeln(-1);

end.