A second expedition had to be conducted to bring Lan Na under control again. The Siamese army invaded Chiang Mai, catching the Burmese command completely off guard. The Siamese captured Chiang Mai on 10 February 1663, and drove back Burmese forces that arrived belatedly at Sai Yok. In November 1663, the Siamese then led an army of 60,000 in an invasion of Burma, capturing Martaban, Moulmein, Syriam, Rangoon, Pegu, and then laid siege to Pagan in 1664. Burmese defences withstood several Siamese onslaughts until May 1664 before the rainy season arrived, after "causing many casualties in dead and wounded and capturing many prisoners of war", the Siamese retreated.:220-239 Meanwhile, the Siamese garrison at Chiang Mai was holed up in a deserted city, and its troops were constantly ambushed by resistance forces whenever they ventured out of the city. In late November 1664, the Siamese evacuated Chiang Mai, and returned.
Question: How many months did the Saimese stay holed up in Chiang Mai after they captured it?
Answer: 21