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.
['t', '6573', 'K', 'b', '6409', 'S', '9363', 'R', 'Q', 'z', 'y', '2099', '6365', '5087', '8199', '6799', 'P', '6001', 'P']