Note that, since the tourists only move south and east, any grid positions
west or north of the source are unusable. Similarly, any grid positions south
or east of the sink are unusable, so we can simply say that the source vertex
is at (0, 0) and that the sink vertex at (n,m) defines the southeasternmost
corner of the grid. In figure 6.4 n = m = 4, but n does not always have
to equal m. We will use the grid shown in figure 6.4, rather than the one
corresponding to the map of Manhattan in figure 6.3 so that you can see a
nontrivial example of this problem.