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.
['6901', 'O', 's', 'E', '5499', '5859', '21', '1997', '7213', 'H', 'f', 'S', 'F', '2799', 'A', '555', 'Z', 's', '3423']