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.
['P', 'Y', 'n', '7403', 'U', '9855', '2877', '5193', '2589', 'D', '5179', '4683', 't', '6119', 'l', '1515', 'b', '1623']