Farmer John's N cows (1 <= N <= 2000) are all standing at various positions along the straight path from the barn to the pasture, which we can think of as a one-dimensional number line. Since his cows like to stay in email contact with each-other, FJ wants to install Wifi base stations at various positions so that all of the cows have wireless coverage.
After shopping around, FJ learns that the cost of a Wifi base station depends on distance it can transmit: a base station of power r costs A + B*r, where A is a fixed cost for installing the base station and B is a cost per unit of transmission distance. If FJ installs such a device at position x, then it can transmit data to any cow located in the range x-r ... x+r. A base station with transmission power of r=0 is allowed, but this only provides coverage to a cow located at the same position as the transmitter.
Given the values of A and B, as well as the locations of FJ's cows, please determine the least expensive way FJ can provide wireless coverage for all his cows.
给出在同一条直线上的n个点和两个数A，B，现在要在这条直线上放置若干个信号塔，每个信号塔有一个r值，假设它的位置是x，则它能覆盖的范围是x-r~x+r，放置一个信号塔的花费是A+B*r，问要覆盖所有的点最小的花费是多少。