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', '479', 'u', '3489', 'o', '9589', '7153', '7649', '123', 'a', 'A', '3951', '847', 'N', 'R', '947', '9243', 'G', 'a']