In April 1400, Li Jinglong mobilised 600,000 men and began advancing northwards toward the Baigou River. On 24 April, the Yan forces engaged with the government forces in a decisive battle.The government forces ambushed Zhu Di, and the Yan forces suffered a series of defeat initially. Landmines were placed in the retreat path of the Yan forces by the government forces, which inflicted heavy losses for the Yan Army on their way back to the camp.A new battle ensued the following day, and the government forces were successful in attacking the rear of the Yan forces. Zhu Di led a personal charge against the main force of Li Jinglong, and the battle turned into a stalemate as Zhu Gaochi arrived with reinforcements.At this point, the wind started to blow and snapped the flag of Li Jinglong in half, which led to chaos in the government camp. Zhu Di seized the opportunity and launched a general assault and defeated the government forces. More than 100,000 government troops surrendered to the Yan forces, and Li Jinglong retreated back to Dezhou once again. On 27 April, the Yan forces began marching toward Dezhou to siege the city. The Yan forces captured Dezhou on 9 May, and Li Jinglong was forced to flee to Jinan. The Yan forces followed up immediately and encircled the city of Jinan on 15 May, and Li Jinglong fled back to Nanjing. Despite of losing the entire army and being condemned by the imperial court, Li Jinglong was spared of execution.

How many days after the Yan forces began marching toward Dezhou did they capture did the city?
12