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.
['6799', 'i', 'z', '5301', '413', '2657', '9665', '3747', 'R', '6671', '337', 'j', 'q', 'K', '3743', 'j', 'b', 'H', 'x', 'S']