def METHOD_NAME(n, d):
    r"""
    Returns the Johnson-radius for the code length `n` and the minimum distance `d`.
    The Johnson radius is defined as `n - \sqrt(n(n-d))`.
    INPUT:
    - ``n`` -- an integer, the length of the code
    - ``d`` -- an integer, the minimum distance of the code
    EXAMPLES::
        sage: sage.coding.guruswami_sudan.utils.johnson_radius(250, 181)
        -5*sqrt(690) + 250
    """
    return n - sqrt(n*(n-d))