In 1990 just over half the cocaine imported into the U.S. came through Mexico, by 2007 that had risen to more than 90 percent, according to U.S. State Department estimates. Although violence between drug cartels has been occurring long before the war began, the government used its police forces in the 1990s and early first decade of the 21st century with little effect. That changed on December 11, 2006, when newly elected Mexican President Felipe Calderón sent 6,500 federal troops to the state of Michoacán to put an end to drug violence there. This action is regarded as the first major retaliation made against cartel operations, and is generally viewed as the starting point of the Mexican Drug War. As time progressed, Calderón continued to escalate his anti-drug campaign, in which there are now well over 25,000 troops involved. During the Felipe Calderón, the Mexican government has spent approximately US$7 billion in an 18-month-old campaign against drug cartels. It is estimated that during 2006, there were about 2,000 drug-related violent deaths, about 2,300 deaths during 2007, and more than 6,200 people by the end of 2008. Many of the dead were gang members killed by rivals or by the government, some have been bystanders.
Question: How many troops did President Felipe Calderón add between sending troops to Michoacán  and the escalation of his anti-drug campaign?
Answer: 18500