What do you mean by successive electron gain enthalpies? Why is the second electron gain enthalpy of an atom positive?

In the following pair, state which atom/ion has higher ionisation enthalpy and more negative electron gain enthalpy?
$\mathrm{I}, {\mathrm{I}}^{-}$

In the following pair, state which atom/ion has higher ionisation enthalpy and more negative electron gain enthalpy.

$\mathrm{Br}, {\mathrm{Br}}^{-}$.

In the following pair, state which atom/ion has higher ionisation enthalpy and more negative electron gain enthalpy? 

$\mathrm{Li}, {\mathrm{Li}}^{+}$

In the following pair, state which atom/ion has higher ionisation enthalpy and more negative electron gain enthalpy?

 $\mathrm{Br}, \mathrm{I}$