def _get_a0(nshell, ncomponent):
    """Return the first mass number in the given shell
    :param nshell: shell (0=s, 1=p, 2=sd, ...)
    :param ncomponent: 1 -> neutrons, 2 -> protons & neutrons
    """
    return int((nshell+2) * (nshell+1) * nshell/3 * ncomponent)