def _Backward1_T_Ps(P, s):
    """Backward equation for region 1, T=f(P,s)

    Parameters
    ----------
    P : float
        Pressure, [MPa]
    s : float
        Specific entropy, [kJ/kgK]

    Returns
    -------
    T : float
        Temperature, [K]

    References
    ----------
    IAPWS, Revised Release on the IAPWS Industrial Formulation 1997 for the
    Thermodynamic Properties of Water and Steam August 2007,
    http://www.iapws.org/relguide/IF97-Rev.html, Eq 13

    Examples
    --------
    >>> _Backward1_T_Ps(3,0.5)
    307.842258
    >>> _Backward1_T_Ps(80,3)
    565.899909
    """
    I = [0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 3, 3, 4]
    J = [0, 1, 2, 3, 11, 31, 0, 1, 2, 3, 12, 31, 0, 1, 2, 9, 31, 10, 32, 32]
    n = [174.78268058307, 34.806930892873, 6.5292584978455, 
        0.33039981775489, -1.9281382923196e-07, -2.4909197244573e-23, -
        0.26107636489332, 0.22592965981586, -0.064256463395226, 
        0.0078876289270526, 3.5672110607366e-10, 1.7332496994895e-24, 
        0.00056608900654837, -0.00032635483139717, 4.4778286690632e-05, -
        5.1322156908507e-10, -4.2522657042207e-26, 2.6400441360689e-13, 
        7.8124600459723e-29, -3.0732199903668e-31]
    Pr = P / 1
    sigma = s / 1
    T = 0
    for i, j, ni in zip(I, J, n):
        T += ni * Pr ** i * (sigma + 2) ** j
    return T
