func solution(startAtDay: String, num: Int) -> String? {
  let days: [String] = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
  guard days.contains(startAtDay) else { return nil }

  let index = (days.firstIndex(of: startAtDay) ?? 0) + 1
  let value = index + num
  let normalizedNo = (value - 1) % 7 + 1

  switch normalizedNo {
  case 1:
    return days[0]
  case 2:
    return days[1]
  case 3:
    return days[2]
  case 4:
    return days[3]
  case 5:
    return days[4]
  case 6:
    return days[5]
  case 7:
    return days[6]
  default:
    return nil
  }
}

if let answer = solution(startAtDay: "Sun", num: 23) {
  print("Day -> ", answer)
}