So
This is a triangular region. So we can switch the order of the sum if we do it carefully.
Can you finish it?
Well, there are no x's anymore. The x and y are just the summing variables.
The main observation at this point is that the sum:
almost is (whether it is x or y doesn't matter, it is just a summing variable). The only thing missing is the 0 term. The sum should go from 0 to , whereas this one starts at 1. But the 0 term is
.
So