Task: Question Answering
Input (English): Passage: On the morning of September 11, 2001, nineteen men hijacked four airliners all bound for California. Once the hijackers assumed control of the airliners, they told the passengers that they had a bomb on board and would spare the lives of passengers and crew once their demands were met - no passenger and crew actually suspected that they would use the airliners as suicide weapons since it had never happened before in history, and many previous hijacking attempts had been resolved with the passengers and crew escaping unharmed after obeying the hijackers. The hijackers - members of al-Qaeda's Hamburg cell - intentionally crashed two airliners into the Twin Towers of the World Trade Center in New York City. Both buildings collapsed within two hours from fire damage related to the crashes, destroying nearby buildings and damaging others. The hijackers crashed a third airliner into the Pentagon in Arlington County, Virginia, just outside Washington D.C. The fourth plane crashed into a field near Shanksville, Pennsylvania, after some of its passengers and flight crew attempted to retake control of the plane, which the hijackers had redirected toward Washington D.C., to target the White House or the U.S. Capitol. None of the flights had any survivors. A total of 2,977 victims and the 19 hijackers perished in the attacks. Fifteen of the nineteen were citizens of Saudi Arabia, and the others were from the United Arab Emirates , Egypt, and Lebanon. On 13 September, for the first time ever, NATO invoked Article 5 of the North Atlantic Treaty. On 18 September 2001, President Bush signed the Authorization for Use of Military Force Against Terrorists passed by Congress a few days prior. 
Question: How many days after nineteen men hijacked four airliners intentionally crashed two airliners into the Twin Towers of the World Trade Center in New York City did NATO invoke Article 5 of the North Atlantic Treaty for the first time? 
Answer type: Number 
Output (English): 2.