<s>[INST] For a fixed positive integer $n$, let $A_n$ denote the set of all $n$-bit non-negative integers.
Let $B_n$ denote the set of all $n+1$-bit non-negative integers that can be obtained by appending a binary digit to the right of an integer in $A_n$.
How many elements of $B_n$ can be written as the sum of two elements of $A_n$?[/INST]