Question 181
Let $n$ be a positive integer. Find, with proof, the least positive integer $d_n$ which cannot be expressed in the form
$$
\sum_{i=1}^{n} (-1)^{a_i} 2^{b_i},
$$
where $a_i$ and $b_i$ are nonnegative integers for each $i$.
$$
\sum_{i=1}^{n} (-1)^{a_i} 2^{b_i},
$$
where $a_i$ and $b_i$ are nonnegative integers for each $i$.