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', '6515', '7323', 'j', 'a', 'j', 't', '3091', '3273', '4661', 'U', '3979', '2481', '2307', '6199', 'H', 'K', '1023', 'l']