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.
['6865', '2439', '6747', '9785', 'a', '9453', 'K', '6275', 'Z', '6517', 'j', '8283', '4399', 'O', '6545', '6263', '2785', '4625']