C# code using dictionary

int t = Convert.ToInt32(Console.ReadLine()); //int t = 1;

        for (int a0 = 0; a0 < t; a0++)
        {
            string s = Console.ReadLine();

            if (s.Length % 2 != 0) { Console.WriteLine(-1); continue; }
            Dictionary<char, int> l = new Dictionary<char, int>();

            int half = s.Length / 2;
            for (int i = 0; i < half; i++)
            {
                if (i < half)
                {
                    if (l.ContainsKey(s[i])) l[s[i]]++;
                    else l.Add(s[i], 1);
                }
            }

            int qtyDiff = 0;
            for (int j = half; j < s.Length; j++)
            {
                if (l.ContainsKey(s[j]) && l[s[j]] > 0) l[s[j]]--;
                else qtyDiff++;
            }

            Console.WriteLine(qtyDiff);
        }