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.
['i', 'O', '7881', 'Y', 's', 'G', '7445', '6671', '5415', 'B', '2129', '9493', '3039', 'r', 'F', 'b', '2499', 'N']