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.
['6741', 'N', '6539', 'g', 'o', 'U', 'S', '3007', '8575', 'i', 'L', 'V', 'y', '3979', '2417', 'm', '7635', '2985', 'c']