12. As shown in the figure, $A$ is the post office, and $B, C, D, E, F$ are five households. The distances between adjacent households are marked in the figure. The mailman starts from the post office to deliver letters to these 5 households (each household has a letter), and is required to deliver the last letter to household $D$. Question: What is the shortest distance the mailman can walk in meters?
12. As shown in the figure, $A$ is the post office, and $B, C, D, E, F$ are five households. The distances between adjacent households are marked in the figure. The mailman starts from the post office to deliver letters to these 5 households (each household has a letter), and is required to deliver the last letter to household $D$. Question: What is the shortest distance the mailman can walk?

【Analysis】According to the problem, the shortest route is $A \rightarrow B \rightarrow C \rightarrow F \rightarrow E \rightarrow D$, from which the problem can be solved.
【Solution】Solution: According to the problem, the shortest route is $A \rightarrow B \rightarrow C \rightarrow F \rightarrow E \rightarrow D$, and the shortest distance is $100+100+100+100+100=500$ meters.

Answer: The shortest distance the mailman can walk is 500 meters.