<|system|>
Compute $\arccos \left( -\frac{1}{2} \right).$  Express your answer in radians.</s>.
<|user|>
Can you answer based on context?
<|assistant|>
Since $\cos \frac{2 \pi}{3} = -\frac{1}{2},$ $\arccos \left( -\frac{1}{2} \right) = \frac{2 \pi}{3}.$