Stone didn't see the formula de either, but as far as your problem is concerned, there is no need for a formula. In both cases, there is a horizontal line on the letter. If you think letters are three-dimensional, you add a horizontal line on the diagonal. That is, some people say that the function of distinguishing 2 is on it. If it is a plane with horizontal lines on it, it means that the average is read as z, and if it is like the letter z, it is really not easy to get it, because the sum function looks like this letter to me.