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.
['h', 'K', 'K', '6779', '2837', '507', '8481', 'a', 'Z', '5757', '2971', 'a', '6389', '8959', '1197', '9399', 's', 'V', 't', '5321']