The United States occupation of Haiti began on July 28, 1915, when 330 US Marines landed at Port-au-Prince, Haiti, on the authority of US President Woodrow Wilson. The first invasion forces had already disembarked from USS Montana on January 27, 1914. The July intervention took place following the murder of dictator President Vilbrun Guillaume Sam by insurgents angered by his political executions of elite opposition. The occupation ended on August 1, 1934, after President Franklin D. Roosevelt reaffirmed an August 1933 disengagement agreement. The last contingent of US Marines departed on August 15, 1934, after a formal transfer of authority to the Garde d'Haïti.
Question: How many years after the invasion forces disembarked the USS Montana did the occupation end?
Answer: 20