I am doing an analysis on the complexity of some set-related algorithm where the input is a random set. One of my setbacks can be formulated as follows:

Pick $k$ distinct numbers out of numbers $[1,n]$ uniformly at random, order them increasingly by size and denote them with $\alpha_1, \alpha_2, \dots , \alpha_k$.

I would like to calculate $$\mathrm{E}\left(\sum_{i=1}^{k}\left(\frac{1}{2}\right)^{\alpha_{i}-i}\right),$$ or at least find a good upperbound to it.