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.
['4381', '7145', '5675', '3171', '8805', 'r', 'W', 'C', 'x', 'O', '9143', 'u', '3797', '6571', 'E', '1311', '7995', '6989', '8409']