On 17 December 1297, the three brothers overthrew Kyawswa, and founded the Myinsaing Kingdom. At Pagan, Kyawswa's son Sawhnit was elected king by the dowager Queen Saw but soon became a governor under the authority of Myinsaing. Another of Kyawswa's sons, Kumara Kassapa, escaped to China. The Mongols did not know about the dethronement until June-July 1298. In 1300, the Myinsaing forces led by Athinkhaya attacked the Mongol garrisons north of Mandalay named Nga Singu and Male. On 22 June 1300, the Mongol Emperor declared that Kumara Kassapa was the rightful king of Burma, and sent in an army from Yunnan. The invasion force reached Myinsaing on 25 January 1301 but could not break through. The besiegers took the bribes by the three brothers, and withdrew on 6 April 1301. The Mongol government at Yunnan executed their commanders but sent no more invasions. They withdrew entirely from Upper Burma starting on 4 April 1303. By then, the city of Pagan, once home to 200,000 people, had been reduced to a small town, never to regain its preeminence.  The brothers placed one of Kyawswa's sons as the governor of Pagan. Anawrahta's line continued to rule Pagan as governors under Myinsaing, Pinya and Ava kingdoms until 1369. The male side of Pagan ended there although the female side passed into Pinya and Ava royalty. But the Pagan line continued to be claimed by successive Burmese dynasties down to the last Burmese dynasty Konbaung.
Question: How many years after the overthrow of Kyawswa did the Mongol Emperor declared that Kumara Kassapa was the rightful king of Burma?
Answer: 3