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.
['4621', 'n', '3409', '9435', 'I', 'F', 'o', '4431', 'W', 'y', '9979', 'S', '6235', 'y', 'F', 'm', '5473', 'X', '8647']