The owner of a local jewellery store hired 3 watchmen to guard his diamonds, but a thief still got in and stole some diamonds. On the way out the thief met each watchman, one at a time. To each he gave `1/2` of the diamonds he had then, and 2 more besides. He escaped with one diamond. How many did he steal originally? (a) 25 (b) 36 (c) 40 (d) None of these