How do we add and increment Javascript numbers?

Here are some examples:

var SLIDEindex = 1;
SLIDEindex = SLIDEindex + 1;  // increment by one
SLIDEindex++;                 // This is short hand for increment by 1

We can do things like add and subtract numbers together, divide, multiply etc.

var DIGITone = 100;
var DIGITtwo = 200;

var SUMtotal = DIGITone + DIGITtwo;