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.
['6439', '2637', 'g', 'q', 'S', '6121', 'i', 'x', 'O', 'd', '6835', '9209', 'Q', 'p', 'I', '4559', '2215', '7407', 'g']