In the permutation and combination with natural numbers as elements, the calculation method of the "distance limit" problem (the difference between adjacent elements gives a limit condition) is much more difficult than the common additional condition problem.

It is not only an excellent subject in mathematics competition, but also a valuable subject in the study of initial numbers.

Distance generally refers to the line segment between two points. In mathematics, distance is a scalar, has no direction and cannot be negative. At the same time, distance is one of the most basic concepts in functional analysis. Intuitively, if the sequence is regarded as a series of points on the real number axis, then the distance between any two points is equal to the absolute value of the difference between the two points.