public String licenseKeyFormatting(String S, int K) {

String[] groups = S.split("-");

String characters = String.join("", groups).toUpperCase();

StringBuilder result = new StringBuilder();

for (int index = characters.length() - 1, counter = 0 ; index >= 0 ; index--, counter++) {

if (counter % K == 0 && counter != 0) {

result.append('-');

}

result.append(characters.charAt(index));

}

return result.reverse().toString();

}