Hamilton Avenue has eight houses. On one side of the street are the houses numbered $1, 3, 5, 7$ and directly opposite are houses $2, 4, 6, 8$ respectively. An eccentric postman starts deliveries at house $1$ and delivers letters to each of the houses, finally returning to house $1$ for a cup of tea. Throughout the entire journey he must observe the following rules:  
1. The numbers of the houses delivered to must follow an odd-even-odd-even pattern throughout.  
2. Each house except house $1$ is visited exactly once (house $1$ is visited twice).  
3. The postman at no time is allowed to cross the road to the house directly opposite.  

How many different delivery sequences are possible?

Let's think step by step and output the final answer within \boxed{}.
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