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.
['z', 't', 'u', 'a', 'V', 'c', '4923', 'N', 'I', '6901', 'J', '4287', 's', 'h', 'b', '6421', 'a', '3963', 'P']