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.
['581', 'm', 'o', 'd', 'n', '4933', '7291', '6015', 'f', '9553', '6679', '21', 'L', 'c', 'N', '5661', 'Y', 'T', '3799']