def p2_p1(M1, gamma):
    """Pressure ratio across a normal shock (eq. 3.57)

    :param <float> M1: Mach # before the shock
    :param <float> gamma: Specific heat ratio

    :return <float> Pressure ratio p2/p1
    """

    return 1.0 + 2.0 * gamma / (gamma + 1.0) * (M1 ** 2 - 1.0)