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', '439', 'N', 'Z', 'r', '3875', '9629', 'A', 'p', '7409', '8039', '3791', '5077', 'g', '7041', 'u', 'U', 'V']