Cars parked in the parking lot problem

There are many cars parked in the parking lot. The parking is a straight very long line and a parking slot for every single meter. There are cars parked currently and you want to cover them from the rain by building a roof. The requirement is that at least k cars are covered by the roof. What's the minimum length of the roof that would cover k cars?

The function has the following parameters:
cars: integer array of length denoting the parking slots where cars are parked
k: integer denoting the number of cars that have to be covered by the roof

Example:
Input:
cars: [2, 10, 8, 17]
k: 3
output: 9
Explanation: you can build a roof of length 9 covering all parking slots from the 2nd one to the 10th one, so covering 3 cars at slots 2, 10, 8, there are no shorter roof that can cover 3 cars, so the answer is 9

Note:
1 <= n <= 10^5
1 <= k <= n
1 <= cars[i] <= 10^14
All slots token by cars are unique