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.
['O', 'o', '4257', '9261', '2733', 'a', '1811', 'f', '6715', '1205', '6561', '715', 'P', 'N', '9831', '1375', '4009', '9213']