Kabuls population is estimated to have been about 10,000 in 1700, 65,000 by 1878, and 120,000 by 1940. More recently, the population was around 500,000 in 1979, whilst another source claims 337,715 as of 1976. This figure rose to about 1.5 million by 1988, before dramatically dropping in the 1990s. Kabul became one of the fastest growing cities in the world, with its population growing fourfold from 2001 to 2014. This was partly due to the return of refugees after the fall of the Taliban regime, and partly due to Afghans moving from other provinces mainly due to war between Taliban insurgents and Afghan government forces in their native areas as well as looking for labor. This resulting rapid urbanization mean that many residents today live in informal settlements. Shanty mud-brick homes on the mountainsides and steep hills have been built by them and these are usually poverty-stricken, not connected to the water and electricity grid. Although the settlements are illegal, they have been tolerated by authorities. In 2017 Kabul Municipality started a project to paint the homes in these settlements in bright colors in an effort to "cheer up" residents.
Question: How many years did it take for the population to grow four-fold?
Answer: 13