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.
['8013', 'E', '4749', '9437', '1999', 'D', 'n', 'X', '4457', '3581', 'O', 'M', 'L', '7715', 'f', 'Y', '6123']