You are given two binary strings (s) and (t), each of the same length (n). You are allowed to perform the following operation: Pick indices (l), (r) ((1 \le \color{red}{l < r} \le n)) such that substring (s_{l,r}) is a palindrome and flip all bits in substring (s_{l,r}). The goal is to finally make (s) equal to (t), performing any of the above operations at most (2n) times (possibly none). A substring (s_{l,r}) of a string (s) is the contiguous sequence of characters starting from index (l) and ending at index (r) (both inclusive), where (1 \leq l < r \leq |s|). Here (|s|) denotes the length of the string (s). A string is a palindrome if it reads the same forwards and backwards. For example, the strings 101 and 00 are palindromes, while 10 is not. Flipping all bits in a substring means changing each 0 to 1 and each 1 to 0 in that substring. For example, flipping the substring 101 results in 010 . Each test contains multiple test cases. The first line contains the number of test cases (T) ((1 \le T \le 5\cdot 10^3)). The description of the test cases follows. The first line of each test case contains an integer (n) ((4 \le n \le 100)) — the length of the strings (s) and (t). The next two lines of each test case contain the binary strings (s) and (t), respectively. It is guaranteed that the sum of (n^2) over all test cases does not exceed (5\cdot 10^5). For each test case, if it is impossible to achieve the goal, print (-1). Otherwise, the first line should contain an integer (k) ((0 \leq k \leq 2n)) — the number of operations. For each of the next (k) lines, print two integers (l, r) ((1 \leq l < r \leq n)) — the indices you choose in each operation. Note that (s_{l,r}) must be a palindrome at this stage. For the first test case: Initially, (s = \mathtt{01011}) and (t = \mathtt{10000}). First, we choose $$$l=1$$