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.
['c', 'f', 'n', 'I', 'n', '7747', 'N', '1461', '1573', 'j', '3361', '5709', '4337', 'g', 'd', 'S', '4187', 'd', '6631']