Article: The main international airport serving Kathmandu and thus Nepal is the Tribhuvan International Airport, located about six kilometers (6 km (3.7 mi)) from the city centre. Operated by the Civil Aviation Authority of Nepal it has two terminals, one domestic and one international. At present, about 22 international airlines connect Nepal to other destinations in Europe, Asia and the Middle East, to cities such as Istanbul, Delhi, Kolkata, Singapore, Bangkok, Kuala Lumpur, Dhaka, Islamabad, Paro, Lhasa, Chengdu, and Guangzhou. A recent extension to the international terminal has made the distance to the airplanes shorter and in October 2009 it became possible to fly directly to Kathmandu from Amsterdam with Arkefly. Since 2013, Turkish Airlines connects Istanbul to Kathmandu. Regionally, several Nepali airlines operate from the city, including Agni Air, Buddha Air, Cosmic Air, Nepal Airlines and Yeti Airlines, to other major towns across Nepal.

Now answer this question: Starting in the center of Kathmandu, how many kilometers must one travel to reach Tribhuvan International Airport?
6