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.
['779', 'r', '6853', 'U', 'j', 'W', 'n', 'A', 'g', '4203', '3189', '687', '9719', '1291', 'N', '7995', '6049', 'l']