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', 'j', '9993', '7303', '1539', '1977', '6213', 't', 't', 'F', '797', 'f', 'E', 'f', 'm', 'y', '2099', '1995', '3667']