Point \(B\) is on \(\overline{AC}\) with \(AB = 9\) and \(BC = 21.\) Point \(D\) is not on \(\overline{AC}\) so that \(AD = CD,\) and \(AD\) and \(BD\) are integers. Let \(s\) be the sum of all possible perimeters of \(\triangle ACD.\) Find \(s.\)

(第二十一届AIME1 2003 第7题)