This circuit implements a two-bit adder, where each input has one bit.

`A + B + Cin = S with Cout`

While there is an OR gate in the design, there are several other [logic gates](/digital_design/logic_gates) too. **Using the truth table below, can you make sense of the other logic gates?**

| Cin     | A       | B      | S      | Cout   |
|---------|---------|--------|--------|--------|
| 0       | 0       | 0      | 0      | 0      |
| 0       | 0       | 1      | 1      | 0      |
| 0       | 1       | 0      | 1      | 0      |
| 0       | 1       | 1      | 0      | 1      |
| 1       | 0       | 0      | 1      | 0      |
| 1       | 0       | 1      | 0      | 1      |
| 1       | 1       | 0      | 0      | 1      |
| 1       | 1       | 1      | 1      | 1      |

**Extra credit: using this circuit, can you build a four-bit full adder?** This circuit needs to accept two two-bit values, A and B, and a carry-in bit. It needs to output a two-bit value, S, and a carry-out bit.

Feel free to play around to try to figure it out. You can always return to the [logic gate tutorial](/digital_design/logic_gates) if you need a refresher. 

You can also check out the [**solution**](https://wokwi.com/projects/344249305917293138).