More max mana with armor

I need to make magic armors for my mod.
They should give more max mana when equipped, so I need procedure that give more max mana when armor piece is equipped and change mana back to normal when armor is unequipped.

There will be at least 5 magic armor sets and I don't want to make whole sets giving more max mana, every piece of armor should work separately.

(I made mana with variables)

So for example for one of hats procedure should:

give +2 max mana if armor piece is equipped

give -2 max mana if armor piece is unequipped