I'm a bit confused by this question.
Let X have a uniform distribution on $(-1,1)$ and let Y be independent of X with a uniform distribution over $(-1,1)$. What is the cumulative distribution function of $S=\sqrt{X^2+Y^2}$.
In other words, if you make a move in the X and Y direction (uniformly distributed), what is the distribution of the total distance travelled?
Kind regards