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.
['7685', '5587', 'k', 'p', '1421', 'd', 'e', 'k', 'R', 'k', 'j', '2531', '3207', '3573', 'r', '4043', '5589', 'z']