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.
['l', '5369', 'z', 'U', 's', '2357', '3367', 't', '6539', 'K', 'a', 'Q', 'x', 'e', '6459', 'K', 'B', 'f', 'j']