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.
['U', 'j', '9411', 'R', '3921', '7563', 'n', '4031', 'a', '9491', 'T', 'g', 'B', 'U', '571', '7431', 'r', '1107', '7427']