The standard deviation of Y is 0.6. You square it to get the variance. That's 0.36. You add these two up, and you are going to get one. So the variance of the sum is one. And then if you take the square root of both of these, you get the standard deviation of the sum is also going to be one. And that just happened to work out because we're dealing with the scenario where the variance, where the square root of one is, well, one.