The following example:
#set($thisCampNumber = 10)
#set($nextCampNumber = $thisCampNumber+1)
#set($previousCampNumber = $thisCampNumber-1)
#set($previousCampNumberB = $thisCampNumber - 1)
produces this result
Note that using a minus sign in a #set statement does not work if there are no spaces around it. (however, the same is not true for +).