Assumption is all you need

JB holds the belief that assumption is all you need to solve a problem. To prove it, JB gave you two permutations $A$ and $B$ of the numbers $1$ to $n$. JB wants a sequence of element swap operations $(x_i, y_i)$ on $A$ such that:

1. Every swapped pair forms an inversion when its operation is performed. That is, $x_i < y_i$ and $A_{x_i} > A_{y_i}$ at the moment of the $i$-th operation.
2. $A$ becomes $B$ after the last operation.

If no such sequence exists, determine that it is impossible. Help prove JB's belief by solving this problem!

The first line contains one integer $T$, the number of test cases. Each test case is given as follows.

The first line contains one integer $n$ ($1 \le n \le 2021$), the number of elements in $A$ and $B$.

The second line contains $n$ distinct integers $A_1, A_2, \dots, A_n$ ($1 \le A_i \le n$), the array $A$.

The third line contains $n$ distinct integers $B_1, B_2, \dots, B_n$ ($1 \le B_i \le n$), the array $B$.

The sum of $n$ over all test cases does not exceed $2021$.

For each test case, if no sequence exists, print one line containing the integer -1.

Otherwise, print the number of operations $k$ ($0 \le k \le \frac{n(n-1)}{2}$) on the first line. Then print $k$ lines, each with two integers $x_i$ and $y_i$ ($1 \le x_i < y_i \le n$), describing the $i$-th operation $\text{swap}(A_{x_i}, A_{y_i})$.