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', 'M', 'v', 'I', '4333', '5867', '1909', '8455', '3143', '5547', '3725', 'U', '171', '7475', 'I', 'v', 'B', 'a', '5397']