An x-path from the bottom squares of the grid to the top squares of the grid considitues a seqence of horizontally or vertically adjacent x-squares from some bottom x-square to some top x-square. Similar with o-paths.
An exit is a top square that is connected to a bottom square by either an x-path or an o-path. For example, in the 8 by 3, x/o grid
x x o o x x o o x x o o x o x x x x o o x x x o x o x
has two o-exits and three x-exits for a total of five exits.
In an n by m, x/o grid, what is the expected number of exits?