import math


def pi(var_lambda: float, gamma=1.4) -> float:
    """"
    Returns gas dynamic function pi

    Parameters
    ----------
    var_lambda: float
        superficial velocity of flow
    gamma: float
        isentropic exponent (default 1.4)

    Returns
    -------
    pi: float
        values of GD function
    """
    gp1 = gamma + 1.0
    gm1 = gamma - 1.0

    power = gamma / gm1

    return math.pow(1.0 - (gm1 / gp1) * var_lambda*var_lambda, power)