Lets say that the vertical line in Y and Horizontal line in X
Take any point from either the number 8 diagonal or number 17 diagonal and see the value it has on both X and Y line and sum them , u will find the sum is always the same , this is because all the squares made by closest numbers r the same as any of the squares in this , think this as a graph paper that makes more sense if u r trying to understand ....
