private int compareStrings(String s1, String s2) {
//        if (s1 == null || s2 == null)
//            return -1;
//            
//        if (s1.length() !=  s2.length())
//            return -1;
            
        int difference = 0;
        for (int i = 0; i < s1.length(); i ++) {
            if (s1.charAt(i) != s2.charAt(i))
                difference ++;
        }
        
        return difference;
    }