B'You are given a string s , consisting of lowercase English letters. In one operation, you are allowed to swap any two characters of the string s . A string s of length n is called an anti-palindrome, if s[i] ne s[n - i + 1] for every i ( 1 <= i <= n ). For example, the strings "codeforces", "string" are anti-palindromes, but the strings "abacaba", "abc", "test" are not. Determine the minimum number of operations required to make the string s an anti-palindrome, or output -1 , if this is not possible. The first line contains a single integer t ( 1 <= t <= 10^4 ) -- the number of test cases. The description of the test cases follows. Each test case consists of two lines. The first line contains a single integer n ( 1 <= n <= 2 cdot 10^5 ) -- the length of the string s . The second line contains the string s , consisting of n lowercase English letters. The sum of n over all test cases does not exceed 2 cdot 10^5 . For each test case, output a single integer -- the minimum number of operations required to make the string s an anti-palindrome, or -1 if this is not possible. In the first test case, the string "codeforces" is already an anti-palindrome, so the answer is 0 . In the second test case, it can be shown that the string "abc" cannot be transformed into an anti-palindrome by performing the allowed operations, so the answer is -1 . In the third test case, it is enough to swap the second and the fifth characters of the string "taarrrataa", and the new string "trararataa" will be an anti-palindrome, so the answer is 1 . '...