The command processor CMD.EXE comes with a mini-calculator that can perform simple arithmetic on 32-bit signed integers:

C:\>set /a 2+2 
4 

C:\>set /a 2*(9/2)
8

C:\>set /a (2*9)/2
9

C:\>set /a "31>>2"
7 

Note that we had to quote the shift operator since it would otherwise be misinterpreted as a "redirect stdout and append" operator.

For more information, type set /? at the command prompt.

Source: http://blogs.msdn.com/oldnewthing/archive/2006/05/04/589884.aspx