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.
['579', '9839', 'z', '9557', '1657', '6319', 'm', 'K', 'F', 'L', 'k', 'q', 'r', '4703', '921', '7765', '9367', 'C']