You will be given a definition of a task first, then some input of the task.
This task involves creating answers to complex questions, from a given passage. Answering these questions, typically involve understanding multiple sentences. Make sure that your answer has the same type as the "answer type" mentioned in input. The provided "answer type" can be of any of the following types: "span", "date", "number". A "span" answer is a continuous phrase taken directly from the passage or question. You can directly copy-paste the text from the passage or the question for span type answers. If you find multiple spans, please add them all as a comma separated list. Please restrict each span to five words. A "number" type answer can include a digit specifying an actual value. For "date" type answers, use DD MM YYYY format e.g. 11 Jan 1992. If full date is not available in the passage you can write partial date such as 1992 or Jan 1992. If you find multiple spans, please add them all as a comma separated list. Please restrict each span to five words.

Passage: Abdali invaded the Mughal Empire seven times from 1748 to 1767. According to Jaswant Lal Mehta, Durrani aroused the Afghans "religious passions" to fire and "sword into the land of infidels India." He crossed the Khyber pass in December 1747 with 40,000 troops for his first invasion of India. He occupied Peshawar without any opposition. He first crossed the Indus River in 1748, the year after his ascension- his forces sacked and absorbed Lahore. The following year , the Mughal ruler was induced to cede Sindh and all of the Punjab including the vital trans Indus River to him, in order to save his capital from being attacked by the forces of the Durrani Empire. Having thus gained substantial territories to the east without a fight, Durrani and his forces turned westward to take possession of Herat, which was ruled by Nader Shah's grandson, Shah Rukh. The city fell to the Afghans in 1750, after almost a year of siege and bloody conflict; the Afghan forces then pushed on into present-day Iran, capturing Nishapur and Mashhad in 1751. Durrani then pardoned Shah Rukh and reconstituted Khorasan, but a tributary of the Durrani Empire. This marked the westernmost border of the Afghan Empire as set by the Pul-i-Abrisham, on the Mashhad-Tehran road.
Question: How many years after crossing the Khyber pass did the city of Herat fall?
Answer type: Number
Output:
3.