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.
['4901', '6415', '3247', '8931', 'f', 'g', 'c', 'S', 'R', '339', '643', 'M', 'G', '5015', 'U', '887', '7723', 'n']