Find the number of diagonals that can be drawn in an n-side polygon

Find the number of diagonals that can be drawn in an n-side polygon.

I don't know how to do that.

Set up a recursion for the number of diagonals in an n-gon: $D_{n+1} = D_n + n - 2$.
$D_{n+1} = D_n + n - 2$.