Mine

class IterQuersumme {
  
  public static void main(String[] arg) {
    int qsumme = 0, n = In.readInt();
    do {
      while (n > 0) {
        qsumme += (n % 10);
        n = n / 10;
      }
      n=qsumme;
      qsumme=0;
    } while (n >= 10);
    Out.println(n);
  }
}