You are given an M x N grid of nonnegative integers and want to find a path from the
upper-left corner from the lower-right corner.  You are restricted in your motion,
though, and so you can only walk right and down.  We say that the length of your path
is the sum of the integers in each square that you walk down.  Write an algorithm
that, in O(MN) time, returns the shortest path meeting this criteria.