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.
['U', 'h', '4537', '6839', 'a', 'n', 'C', '5459', 'z', 'm', '9579', '2497', 'b', 'C', '6135', '1565', 'p', '8793']