Suppose I have a small particle and put it on the center of an interval on a 1-D axis. If the particle undergoes a motion that satisfies: 

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> 1. It chooses its direction freely and randomly
> 2. It chooses the span of each step of equal probability in range (0,r]
> 3. The total number of the steps is unlimited
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So I'd like to ask that what's the probability that the particle finally jump out of the interval specified as a function of r.