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.
['4303', 'R', '195', 'u', 'S', '2767', '187', '8901', '9003', '6021', 'g', 'H', 'b', 'X', 'S', 'e', 'a', '6743']