A - Array Balancing

You are given two integer arrays $a$ and $b$ of the same **even** length $n$, where $0 \le a_i \le n$ and $0 \le b_i \le n$ for all $i$. You want to transform array $a$ into array $b$ using the following operation, which you may apply at most $2 \cdot n$ times (possibly zero): Choose a subset $S$ of indices of size exactly $\frac{n}{2}$, that is, $S \subseteq \{1, 2, \ldots, n\}$ and $|S| = \fra