Which MagSafe 2 charger do I need?

Hi,

I have an old Mac Book Pro retina 13" (late 2013) model. I need a new charger which I know is a MagSafe 2 but how do I know the wattage I should buy - there seems to be 45w, 60w & 80w. Can't check old adaptor as writing has rubbed off! Any ideas please?