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.
['3873', 'D', '6749', 'P', '4805', '4379', 'T', '8819', '8275', 'A', '4113', '9345', 'O', 'M', 'L', '5789', 'u', '557', 'C']