Algebra Word Problem

[KandiKane]

Each policeman can see 100 yards in any direction. It takes 2 policemen to patrol a city consisting of a single block of side 100 yards. It takes 3 to police a 2 X 1 block city, and it takes 4 policemen to patrol a 2 X 2 block city. Whats the minimum number of policemen needed for an n X m block city?

 

[Gene]

Some books call it int = integer others call it floor. Both mean drop the decimal part. I'll use {} {2.5}=2
My best guess is
cops = (m+1)+{((m+1)/2)*(n-1)}