Can you replace the question mark with a number?

A man took his Rottweiler to the vet and said "My dog's cross-eyed, is there anything you can do for him?"

"Well," said the vet, "lets have a look at him." So he picks the dog up and has a good look at its eyes.

"Hmm," says the vet, "I'm going to have to put him down"

"Just because he's cross-eyed?" says the man.

"No, because he's heavy," says the vet.