public class Solution {
    public String licenseKeyFormatting(String S, int K) {
        StringBuilder sb = new StringBuilder();
        for(int i=S.length()-1; i>=0; i--) {
            if(S.charAt(i) != '-') {
                sb.append(sb.length() % (K+1) == K ? "-" : "").append(S.charAt(i));
            }
        }
        return sb.reverse().toString().toUpperCase();
    }
}