How to add layer to layer mask?

I don’t quite understand how to work with bitmasks or layer masks yet, so…
How to add one layer to layer mask?
For instance I’ve got public LayerMask and assigned some layers through inspector. Now I want to cast Physics2D.OverlapAreaAll, with that LayerMask, but add one extra layer to it.

yourLayerMask |= (1 << LayerMask.NameToLayer(“Default”));
yourLayerMask |= (1 << LayerMask.NameToLayer(“AnotherLayer”));
yourLayerMask |= (1 << LayerMask.NameToLayer(“Water”));