A12560. Minimum Triangulation

You are given a regular polygon with $n$ vertices labeled from $1$ to $n$ in counter-clockwise order. The triangulation of a given polygon is a set of triangles such that each vertex of each triangle is a vertex of the initial polygon, there is no pair of triangles such that their intersection has non-zero area, and the total area of all triangles is equal to the area of the given polygon. The weight of a triangulation is the sum of weigths of triangles it consists of, where the weight of a triagle is denoted as the product of labels of its vertices.

Calculate the minimum weight among all triangulations of the polygon.

The first line contains single integer $n$ ( $3 \le n \le 500$ ) — the number of vertices in the regular polygon.

Print one integer — the minimum weight among all triangulations of the given polygon.

According to Wiki: polygon triangulation is the decomposition of a polygonal area (simple polygon) $P$ into a set of triangles, i. e., finding a set of triangles with pairwise non-intersecting interiors whose union is $P$ .

In the first example the polygon is a triangle, so we don't need to cut it further, so the answer is $1 \cdot 2 \cdot 3 = 6$ .

In the second example the polygon is a rectangle, so it should be divided into two triangles. It's optimal to cut it using diagonal $1-3$ so answer is $1 \cdot 2 \cdot 3 + 1 \cdot 3 \cdot 4 = 6 + 12 = 18$ .