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.
['5749', 'l', '3297', '8913', '4625', 'M', 'X', 'g', '3143', 'i', '8369', '6101', 'w', '3683', '3891', '1399', '9105', '6923', 'K']