func superDigit(n string, k int32) int32 {
  if len(n) == 1 {
    return int32(n[0]-'0')
  }
  
  var sd int64
  for _, r := range n {
    sd = sd + int64(r-'0')
  }
  
  return superDigit(fmt.Sprintf("%d", sd * int64(k)), 1)
}