I want to replace an existing 3-way switch with a 3-way dimmer. The old switch has 2 brass screws and 2 black screws, plus a green ground screw. Ignoring the ground, there are wires attached to all four screws and all four wires are brown. The dimmer switch has 2 brass screws but only one black screw. How do I connect the 2 wires that are on the 2 black screws on the old switch to the new dimmer switch?