E. Making Anti-Palindromes

#include <bits/stdc++.h>

using namespace std;

int main() {
    ios_base::sync_with_stdio(false), cin.tie(NULL), cout.tie(NULL);

    int testcase;
    cin >> testcase;
    while(testcase--) {
        int n;
        cin >> n;

        string s;
        cin >> s;

        if (n % 2) {
            cout << -1 << '\n';
            continue;
        }

        map <char, int> mm;
        int mx = 0;
        for (int i = 0; i < n; i++) {
            mm[s[i]]++;
            mx = max(mx, mm[s[i]]);
        }

        if (mx > (n / 2)) {
            cout << -1 << '\n';
            continue;
        }

        mm.clear();
        mx = 0;
        int c = 0;
        for (int i = 0; i < (n / 2); i++) {
            if (s[i] == s[n - i - 1]) {
                mm[s[i]]++;
                c++;
                mx = max(mx, mm[s[i]]);
            }
        }

        cout << max(mx, (c + 1) / 2) << '\n';
    }

    return 0;
}