add: uva working directory generator.
 Finish the solution barebone of uva_10020.
fix: b.first - b.second => b.second - b.first
Squashed commit of the following:

commit 0956eca4489950b37e4626af4d0a12b8b146ee8c
Author: James Tien <southp0105@gmail.com>
Date:   Mon Jul 8 10:11:42 2013 +0800

    Adjust output format.
add usaco