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.
['W', 'U', 'r', 'q', '6811', 'k', '1425', 'n', 'U', 'w', '6473', 'O', 'H', 'K', 'y', 'u', '4977', '3373']