\frac{2 \pi}{3}
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Compute $\arccos \left( -\frac{1}{2} \right).$  Express your answer in radians.
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
\frac{2 \pi}{3}
Compute $\arccos \left( -\frac{1}{2} \right).$  Express your answer in radians.
train