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.
['S', '3463', '7571', 'G', '7281', '4307', '955', 'x', 'x', 'f', 'm', 'f', '31', '2783', '5389', 'L', 'A', '7653', '3689']