With just one double relay, you can make an adder capable of performing 1bit additions:
0 + 0 = 0
0 + 1 = 1
1 + 0 = 1
1 + 1 = 0 ( with Carry = 1 )
The circuit design isn’t mine, I found it with a quick google search.
This is the complete schematic of the above circuit.