Question

If you have 100 numbers (1 - 100, including 1 and 100) and you choose 2 of them then do the formula x + y + xy (x being the first number and y being the second) and you replace the 2 numbers chosen with that new number, what are the last possible values for the last number?

example on a smaller scale:
1, 2

1 + 2 + 2 = 5
x + y + xy

The possible values for the last number is 5.

I've gotten around the fact that this is probably a combination problem but I'm lost as to how to start this problem.