/// Swift5 -> Swift2.2

extension String {
    // split(separator:maxSplits:omittingEmptySubsequences:)
    func split(separator separator: Character) -> [String] {
        return self.characters.split(separator).map(String.init)
    }
}
extension Array {
    // init(repeating:count:)
    init(repeating: Element, count: Int) {
        self.init(count: count, repeatedValue: repeating)
    }
    // lastIndex(where:)
    func lastIndex(where predicate: (Element) -> Bool) -> Int? {
      	for i in stride(from: count - 1, through: 0, by: -1) {
          	if predicate(self[i]) {
              	return i
            }
        }
      	return nil
    }
}
extension Array where Element: Comparable {
    // sorted()
    func sorted() -> [Element] {
        return self.sort( { $0 < $1 } )
    }
    // max()
    func max() -> Element? {
        guard self.count > 0 else { return nil }
        return self.reduce(self.first!) { return Swift.max($0, $1) }
    }
}
extension String {
    // Sequence#enumerated()
  	// public func enumerate<Seq : SequenceType>(base: Seq) -> EnumerateSequence<Seq>
    func enumerated() -> EnumerateSequence<[Character]> {
        return Array(self.characters).enumerate()
    }
}
// stride(from:through:by:)
func stride(from start: Int, through end: Int, by stride: Int) -> StrideThrough<Int> {
    return start.stride(through: end, by: stride)
}
// stride(from:to:by)
func stride(from start: Int, to end: Int, by stride: Int) -> StrideTo<Int> {
    return start.stride(to: end, by: stride)
}

/// end


let N = Int(readLine()!)!

print(N >= 1000 ? "ABD" : "ABC")
