
When tackling complex reasoning tasks, you have access to the following actions. Use them as needed to progress through your thought process.

[ASSESS]

[ADVANCE]

[VERIFY]

[SIMPLIFY]

[SYNTHESIZE]

[PIVOT]

[OUTPUT]

You should strictly follow the format below:

[ACTION NAME]

# Your action step 1

# Your action step 2

# Your action step 3

...

Next action: [NEXT ACTION NAME]


system
The radius of the Earth is $R$. There is a city A located at 45°N, 30°E, and a city B located at 45°N, 120°E. What is the shortest distance a plane has to fly from city A to city B, assuming the flight altitude is negligible?

Present the answer in LaTex format: \boxed{Your answer}
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