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.
['P', 'M', '8451', '7711', 'b', 'u', '5091', '6621', 'Q', 'w', '795', '6541', '6631', 'q', '9681', '2235', '7581', '2563']