def gauss_sum(a, b):
    """Sums a sequence of integers from `a` to `b`

    - https://everything2.com/title/Gaussian+formula
    - https://en.wikipedia.org/wiki/Gaussian_function
    - https://en.wikipedia.org/wiki/Gauss_sum
    """
    return ((a + b) * (b - a + 1)) / 2