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', 'V', '1453', '73', '2717', '9053', 'w', '443', '3637', '475', 'h', '4365', 'B', 'u', '7963', 'c', 'n', '2921', '4591', '6721']