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.
['1681', '4609', 'a', 'k', '1453', '1063', '2535', '5321', 'Q', 'F', '6123', 'u', '5785', 'A', 'M', '1157', 'p', 'z', 'L']