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', 'R', 'Z', 'S', 'g', '4649', '9119', '713', '4399', '4239', '8901', '2539', '3717', '4209', 'R', '8805', '2475', 'k']