Let $\{X_i\}_{i=1}^N$ be $N$ random variables uniformly distributed over the intervals $[a_i, b_i]$ respectively. How does the sum:
$$\sum_{i=1}^N X_i$$
distribute?
This is a generalization of the Irwin-Hall distribution, which applies when all the intervals $[a_i,b_i]$ are equal.