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.
['5087', '4519', 'V', '6273', '9539', 'H', '2579', 'L', '6991', '6857', '8903', 'y', '4999', 'f', 'P', 'n', 'D']