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.
['5673', '9339', '2719', '6663', '3643', 'P', '4271', '6139', 'N', '1491', '2443', '2115', '8175', 'G', '7405', '3163', '9475', '2021']