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.
['l', '4323', 'L', 'Y', '1621', 'f', 'C', '3501', '1801', 'Y', 'w', '5435', '3373', '483', '8685', '21', 'w', 'j', 'R']