Border dispute

My neighbour came over the other day and said, "Hi, Bob, our houses are identical aren't they?"
"Yes," I said.
"You've recently wallpapered your front room, haven't you?"
"Yes," I said.
"How many rolls of paper did you buy?"
"Twelve," I said.
A few days later I saw him and he said, "I bought 12 rolls of paper and had four left over."
"So did I," I said.