Wrong #defined calculated value

[programandala.net]

I think there's something I'm missing about the way #define values are calculated, look:

Code:

#define screenFirstCol 0
#define screenLastCol 31
#define borderFirstCol screenFirstCol
#define borderLastCol screenLastCol
#define oWinFirstCol  borderFirstCol+1
#define oWinLastCol borderLastCol-1
' Wrong result, 32 instead of 30:
#define oWinWidth oWinLastCol-oWinFirstCol+1

cls
print "Expected values in cramps:"
print
print "screenFirstCol(0)=";screenFirstCol
print "screenLastCol(31)=";screenLastCol
print "borderFirstCol(0)=";borderFirstCol
print "borderLastCol(31)=";borderLastCol
print "oWinFirstCol(1)=";oWinFirstCol
print "oWinLastCol(30)=";oWinLastCol
' Wrong result, 32 instead of 30:
print "oWinWidth(30)=";oWinWidth

pause 0
stop

What's the problem?

I think #defined values are simple labels calculated by the preprocessor. Is it right? Is there any limit about the depth of the calculated values?