function abrir(pagina,ancho,alto) {
var vleft = (screen.width-ancho)/2;
var vtop = (screen.height-alto)/2;
open(pagina,"","toolbar=no,location=no,directories=no,status=no,menubar=no,resizable=no,scrollbars=no,width="+ancho+",height="+alto+",top="+vtop+",left="+vleft);
}