enum ChessColor {
    case black
    case white
}

func chessBoard(x: String, y: Int) -> ChessColor? {
    guard x.count == 1,
    let index = "abcdefgh".range(of: x)?.lowerBound.encodedOffset else {
        return nil
    }

    return (index + y) % 2 == 0 ? .white : .black
}