def sylvester(number: int) -> int:
    assert isinstance(number, int), f"The input value of [n={number}] is not an integer"

    if number == 1:
        return 2
    elif number < 1:
        raise ValueError(f"The input value of [n={number}] has to be > 0")
    else:
        num = sylvester(number - 1)
        lower = num - 1
        upper = num
        return lower * upper + 1