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.
['49', '8927', 'B', '3843', 'k', 'm', 'd', '9973', '1647', '3859', 'n', 'G', '7789', '9803', 'n', 'J', 'U', 'n', 'u', '1863']