"I got a negative two."
"One of the questions was a," stopped listening momentarily here, "so really it was a negative one."
I was quiet for a minute while I tried to figure that out. "Are you saying you got less than none right? One or two less than none? Is that even possible?"
"Mom, it means I got one wrong," he said, exasperated. "Minus one. Maybe you shouldn't be worrying about my math tests."
Maybe not, but I'm concerned for his grammar and sentence structure though.