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.
['9755', '5057', '9601', 'a', '1999', '8137', 'D', 'Z', '2219', 'c', 'L', 'H', 'N', '7947', '345', 'K', 'M', 'U']