3. All the diagonals drawn from one vertex of a convex $n$-sided polygon divide this convex $n$-sided polygon into $m$ small triangles. If $m$ is equal to $\frac{4}{9}$ of the number of diagonals of this convex $n$-sided polygon, then the sum of the interior angles of this $n$-sided polygon is $\qquad$ .
Answer: $720^{\circ}$.