In this task, you are given an input list A. You need to find all the elements of the list that are numbers and calculate their sum.
['2871', '1337', '3771', '4671', 'l', '3785', 'V', 'z', 'R', 'u', '6097', 'R', '4297', 'd', '6339', 'Y', 'V', '5873', '7399']