Task: Question Understanding
Input (English): Passage: The War on Terror, spanning decades, is a multi-trillion-dollar war. According to the Costs of War Project at Brown University's Watson Institute, the War on Terror will have cost $5.6 trillion for operations between 2001-2018 plus anticipated future costs of veterans' care. According to the Soufan Group in July 2015, the U.S. government was spending $9.4 million per day in operations against ISIS in Syria and Iraq. A March 2011 Congressional report estimated war spending through the fiscal year 2011 at $1.2 trillion, and future spending through 2021  at $1.8 trillion. A June 2011 academic report covering additional areas of war spending estimated it through 2011 at $2.7 trillion, and long-term spending at $5.4 trillion including interest.
Question: How many years has the War on Terror gone on for?
Output (English): number