So once again, we took each of the residuals, which you could view as the distance between the points and what the model would predict. We are squaring them. When you take a typical standard deviation, you're taking the distance between a point and the mean. Here we're taking the distance between a point and what the model would have predicted. But we're squaring each of those residuals and adding them all up together. And just like we do with the sample standard deviation, we are now going to divide by one less than the number of residuals we just squared and added. So we have four residuals.