from typing import List


def get_last_parent_index(numbers: List[int]) -> int:
    """
    Gets the last parent index of a list of numbers to convert into a heap.

    The reason we do this is to optimize the algorithm.
    There is no need to recursively call _heapify through the whole list of numbers.
    Rather, we only need to iterate up to the last parent index.

    :param numbers: is the list of numbers.
    :return: the index of the last parent node.
    """
    return (len(numbers) // 2) - 1