who's toady ?
on a serious note technically you should mix the coolant you are adding to the correct strength before adding to the cars cooling system,which the label on the coolant you have bought will tell you how much to mix with water before you put it in.
but its fairly a small amount you are adding so correcting the level with "just coolant" should be ok (but its only my opinion,maybe some one else could confirm or not)
my main concern is where has the coolant gone ?
maybe you have a leak somewhere ?
when did you last check the level ?