MATH PUZZLE: Can you replace...

Paddy and his missus are lying in bed listening to the next door neighbor's dog barking. It had been barking for hours and hours.
Suddenly, Paddy jumps out of bed and says, "I've had enough of this," and goes downstairs.
Paddy finally comes back up to bed and his wife says, "The dog is still barking. What have you been doing?"
Paddy says, "I've put their dog in our yard - now we'll see how they like it!"