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', 'U', '1121', 'q', 'B', 'a', '5761', 'l', '3329', 'K', '7889', '3271', 'C', 'U', '9341', '507', 'Z', 'M']