Read this and answer the question. If the question is unanswerable, say "unanswerable".

Zhu Yousong, however, fared a lot worse than his ancestor Zhu Yuanzhang three centuries earlier. Beset by factional conflicts, his regime could not offer effective resistance to Qing forces, when the Qing army, led by the Manchu prince Dodo approached Jiangnan the next spring. Days after Yangzhou fell to the Manchus in late May 1645, the Hongguang Emperor fled Nanjing, and the imperial Ming Palace was looted by local residents. On June 6, Dodo's troops approached Nanjing, and the commander of the city's garrison, Zhao the Earl of Xincheng, promptly surrendered the city to them. The Manchus soon ordered all male residents of the city to shave their heads in the Manchu queue way. They requisitioned a large section of the city for the bannermen's cantonment, and destroyed the former imperial Ming Palace, but otherwise the city was spared the mass murders and destruction that befell Yangzhou.

When did the Hongguang Emperor leave Nanjing, having been defeated?
late May 1645