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.
['s', '1869', '5665', '3413', 'K', 'I', '9469', 'k', '9051', '3327', '9521', '6481', 'j', 'B', 'l', '2825', '421', '7717', 'S', '4381']