thisiswhy

this works only if the first number is one, because if you imagine a tree that is x by x+1 as given here for 6: all of the squares with a ! in them represent the sigma amount, which is also half of the grid.
 * ||  ||   ||   ||   ||   || a ||
 * ||  ||   ||   ||   || a || a ||
 * ||  ||   ||   || a || a || a ||
 * ||  ||   || a || a || a || a ||
 * ||  || a || a || a || a || a ||
 * || a || a || a || a || a || a ||