#91 · #1701 · Average Waiting Time(平均等待时间)

There is a single chef. Given customers[i] = [arrival_i, time_i], the chef starts cooking customer i's order no earlier than arrival_i and after finishing the previous order, and takes time_i to finish. Return the average waiting time over all customers. Constraints: 1 <= customers.length <= 10^5; customers is sorted by arrival time.