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.
['r', 't', 's', '2781', '8639', '2853', '1133', 'Z', 'y', 'n', '3935', 'o', '7375', 'a', 'Z', '9579', '5015', '1663']