How many combinations will be generated if below conditions are put?
$2^{n-1}-1$

Hint: 1) if you are summing with $k$ numbers $x_1,x_2,.., x_k$ note that once you choose upto $k-1$, last selection is forced on you as the next ascending number to $x_{k-1}$.
2) You are allowed to choose first $k-1$ numbers from n-1 numbers