def _Backward1_P_hs(h, s):
    """Backward equation for region 1, P=f(h,s)

    Parameters
    ----------
    h : float
        Specific enthalpy, [kJ/kg]
    s : float
        Specific entropy, [kJ/kgK]

    Returns
    -------
    P : float
        Pressure, [MPa]

    References
    ----------
    IAPWS, Revised Supplementary Release on Backward Equations for Pressure
    as a Function of Enthalpy and Entropy p(h,s) for Regions 1 and 2 of the
    IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of
    Water and Steam, http://www.iapws.org/relguide/Supp-PHS12-2014.pdf, Eq 1

    Examples
    --------
    >>> _Backward1_P_hs(0.001,0)
    0.0009800980612
    >>> _Backward1_P_hs(90,0)
    91.92954727
    >>> _Backward1_P_hs(1500,3.4)
    58.68294423
    """
    I = [0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 2, 2, 2, 3, 4, 4, 5]
    J = [0, 1, 2, 4, 5, 6, 8, 14, 0, 1, 4, 6, 0, 1, 10, 4, 1, 4, 0]
    n = [-0.691997014660582, -18.361254878756, -9.28332409297335, 
        65.9639569909906, -16.2060388912024, 450.620017338667, 
        854.68067822417, 6075.23214001162, 32.6487682621856, -
        26.9408844582931, -319.9478483343, -928.35430704332, 
        30.3634537455249, -65.0540422444146, -4309.9131651613, -
        747.512324096068, 730.000345529245, 1142.84032569021, -436.407041874559
        ]
    nu = h / 3400
    sigma = s / 7.6
    P = 0
    for i, j, ni in zip(I, J, n):
        P += ni * (nu + 0.05) ** i * (sigma + 0.05) ** j
    return 100 * P
