XOR Sorting

For an array $A$ of length $N$, we define $f(A)$ to be the smallest non-negative integer $K$ such that $A$ can be sorted in ascending order by using the following operation several (possibly, zero) times: - Choose an index $i$ ($1 \le i \lt N$) such that $A_i \oplus A_{i+1} \le K$. - Then, swap $A_i$ with $A_{i+1}$. Here, $\oplus$ denotes the bitwise XOR operation. --- You're given two integers