Why is the error here at most $\frac{1}{3}$?

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From Understanding Machine Learning: Theory and Algorithms:

Can someone explain why the text in the red box below is true? Why does the probability have to be $ \le \frac{1}{3}$? Why can't we have a single region having probability $> \frac{1}{3}$?

The definition of weak learner is included below.


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