import typing
import string


def format_alpha(value: int) -> str:
    """Format a number as lowercase letters a-z, aa-zz, etc."""

    assert value > 0
    result: typing.List[str] = []

    while value != 0:
        value, remainder = divmod(value - 1, len(string.ascii_lowercase))
        result.insert(0, string.ascii_lowercase[remainder])

    return ''.join(result)