Dog math

A farmer is wondering how many sheep he has in his field, so he asks his sheepdog to count them. The dog runs into the field, counts them, and then runs back to his master.
“So,” says the farmer. “How many sheep were there?”
“40,” replies the dog.
“How can there be 40?” exclaims the farmer. “I only bought 38!”
“I know,” says the dog. “But I rounded them up.”