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, -0.183612548787560e2, -0.928332409297335e1,
         0.659639569909906e2, -0.162060388912024e2, 0.450620017338667e3,
         0.854680678224170e3, 0.607523214001162e4, 0.326487682621856e2,
         -0.269408844582931e2, -0.319947848334300e3, -0.928354307043320e3,
         0.303634537455249e2, -0.650540422444146e2, -0.430991316516130e4,
         -0.747512324096068e3, 0.730000345529245e3, 0.114284032569021e4,
         -0.436407041874559e3]

    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