public static int anagram(String s) {
        if (s.length() % 2 != 0) return -1;
        
        int size = s.length() / 2;
        int[] s1 = new int[26];
        int base = 'a';
        int changes = 0;        

        //build first half
        for (int i = 0; i < size; i++) {
            s1[s.charAt(i) - base] ++;
        }

        //check second half
        for (int i = size; i < size*2; i++) {
            if (s1[s.charAt(i) - base] > 0) s1[s.charAt(i) - base]--;
            else changes++;
        }

        return changes;
    
    }