I understand this is either a remarkably stupid or remarkably simple question, but could someone give me an example of a <<set>> macro in which sugarcube does proper arithmetic including various variables?

I've tried a few different ways, but no matter how I do it it simply strings them together. For instance, in an equation where $variable1 is one, $variable2 is two etc: <<set $variable1 to $variable1 + $variable2 + $variable3>> renders $variable1 as 123 instead of 6. Is there a way to write this that would cause it to actually do the math?

I've tried a few different ways, but no matter how I do it it simply strings them together. For instance, in an equation where $variable1 is one, $variable2 is two etc: <<set $variable1 to $variable1 + $variable2 + $variable3>> renders $variable1 as 123 instead of 6. Is there a way to write this that would cause it to actually do the math?