The answer is 0. Since he said the answer is not zero, than the logicle answer would be infinity. It would keep on going and going and there would be no way to solve it. Check and Mate.
There is no 'real' number that you get when you add 'all' numbers together since the number scale is infinate. It will keep on going and going and NEVER ends. So there is no 'real' answer to this question.