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.
['K', 'j', 'g', 'N', 'M', '1643', '3015', 'n', '7931', '4701', 'C', '2579', '1421', 'D', 'I', 'P', 'k', 's', 'o', '5433']