in appendix of spp net, how does it map a window to feature maps?

[李媛媛]

as following picture shows, SPP net  map a window to feature maps by two equation. but i don't understand why subtract 139/2 not 139, and why add 63 later for the top left corner. and then why does equation change for the bottom right corner? the equation 2 add 139/2 and subtract 75, why add 139/2 not subtract? why is 75 not 63? and it adds 1 for  the top left corner but subtract 1 for  bottom right corner?