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.
['7831', 'O', '6017', 'v', 'U', '5761', 'Q', 'Q', 'w', '8465', 'Z', 'P', 'i', 'u', '1635', 'n', '8471', '4213']