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.
['2811', 'l', '6659', 'K', '8661', '4877', '5447', 'm', 't', 'D', '8999', '4723', 'o', '2683', 'R', 'i', 'n', 'l', 'b']