By the end of 1299 count Guy had turned over the government to his eldest son Robert. After the expiration of the armistice in January 1300, the French invaded Flanders again, starting skirmishes alongside the armistice line of 1298. A French detachment led by Wale Paièle plundered and burned the countryside around Ypres and Cassel, and Charles of Valois marched from Bruges to the outskirts of Ghent, burning Nevele and twelve other towns and hamlets. From March 1300 the French besieged Damme and Ypres, where the defenses respectively was led by the count's sons William of Dendermonde and Guy of Namur. At the end of April William surrendered Damme, Aardenburg and Sluis. Ghent surrendered to the French on 8 May, Oudenaarde on 11 May and Ypres on 21 May 1300. By mid May the old count, his sons Robert and William and several Flemish nobles were led into captivity in France, and the whole of Flanders was under French control.
Question: How many cities did William surrender at the end of April?
Answer: 3