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.
['645', '3681', 'Q', 'z', '3549', 'y', 'k', 'K', 'H', '411', '4315', 'p', '2643', 'R', '5943', 'E', 'u', 's', '301', 'L']