Answer:
Basically, a segment which connects two consecutive vertices is not a diagonal, it's actually a side of a polygon.
So, in order to get a diagonal, you need to unit two nonconsecutive vertices, that way you won't get a side but a diagonal.
Actually, a diagonal of a polygon is defined as a segment which connects two nonconsecutive vertices.
Therefore, by definition, a diagonal must connect two nonconsecutive vertices.
