So that would be 1 plus 2 plus 3 plus, and you would go all the way to 99 plus 100. So mathematicians said, well, let's find some notation instead of having to do this dot, dot, dot thing, which you will see sometimes done, so that we can more cleanly express these types of sums. And that's where sigma notation comes from. So this sum up here right over here, this first one, it could be represented as sigma. This is a capital sigma. It's a Greek letter right over here. And what you do is you define an index.