B"You have two strings a and b of equal length n consisting of characters 0 and 1, and an integer k . You need to make strings a and b equal. In one step, you can choose any substring of a containing exactly k characters 1 (and arbitrary number of characters 0) and reverse it. Formally, if a = a_1 a_2 ldots a_n , you can choose any integers l and r ( 1 <= l <= r <= n ) such that there are exactly k ones among characters a_l, a_{l+1}, ldots, a_r , and set a to a_1 a_2 ldots a_{l-1} a_r a_{r-1} ldots a_l a_{r+1} a_{r+2} ldots a_n . Find a way to make a equal to b using at most 4n reversals of the above kind, or determine that such a way doesn't exist. The number of reversals doesn't have to be minimized. Each test contains multiple test cases. The first line contains the number of test cases t ( 1 <= t <= 2000 ). Description of the test cases follows. Each test case consists of three lines. The first line of each test case contains two integers n and k ( 1 <= n <= 2000 ; 0 <= k <= n ). The second line contains string a of length n . The third line contains string b of the same length. Both strings consist of characters 0 and 1. It is guaranteed that the sum of n over all test cases does not exceed 2000 . For each test case, if it's impossible to make a equal to b in at most 4n reversals, print a single integer -1 . Otherwise, print an integer m ( 0 <= m <= 4n ), denoting the number of reversals in your sequence of steps, followed by m pairs of integers l_i, r_i ( 1 <= l_i <= r_i <= n ), denoting the boundaries of the substrings of a to be reversed, in chronological order. Each substring must contain exactly k ones at the moment of reversal. Note that m doesn't have to be minimized. If there are multiple answers,"...