Task: Question Answering
Input (English): Passage: The French king, John II, had been held captive in England. The Treaty of Brétigny set his ransom at 3 million crowns and allowed for hostages to be held in lieu of John. The hostages included two of his sons, several princes and nobles, four inhabitants of Paris, and two citizens from each of the nineteen principal towns of France. While these hostages were held, John returned to France to try and raise funds to pay the ransom. In 1362 John's son Louis of Anjou, a hostage in English-held Calais, escaped captivity. So, with his stand-in hostage gone, John felt honor-bound to return to captivity in England. The French crown had been at odds with Navarre  since 1354, and in 1363 the Navarrese used the captivity of John II in London and the political weakness of the Dauphin to try to seize power. Although there was no formal treaty, Edward III supported the Navarrese moves, particularly as there was a prospect that he might gain control over the northern and western provinces as a consequence. With this in mind, Edward deliberately slowed the peace negotiations. In 1364, John II died in London, while still in honourable captivity. Charles V succeeded him as king of France. On 7 May 1364, one month after the dauphin's accession and three days before his coronation as Charles V, the Navarrese suffered a crushing defeat at the Battle of Cocherel. 
Question: How many citizens from principal towns in France could be held as hostage in lieu of John II? 
Answer type: Number 
Output (English): 18.