from typing import List
from typing import Union


def inverse_cumulative_sum(increasing_numbers: List[Union[int, float]]):
    """ inverse function of cumulative sum

    :param increasing_numbers: ([number]) [1,3,6]
    :return: ([number]) [1,2,3]
    """
    if not increasing_numbers:
        return []

    inv_cumsum = [increasing_numbers[0]]
    for e1, e2 in zip(increasing_numbers[:-1], increasing_numbers[1:]):
        inv_cumsum.append(e2 - e1)

    return inv_cumsum