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.
['1817', '347', '961', 'z', '1417', '7223', 'x', '9869', '6961', 'G', '1251', '6979', '4721', 'R', 'p', 'U', '9015', '4291', '6461', 's']