Counting Domino Spots

Dominoes are pieces used in many tile games. Each domino piece has two marks, and each mark carries $0$ or more spots. The size of the set decides how many spots a mark can carry: in a set of size $N$, every mark carries between $0$ and $N$ spots, inclusive.

Two pieces are the same piece if their two marks carry the same numbers of spots, whatever the reading order. A piece with marks of $2$ and $8$ spots is the same as a piece with marks of $8$ and $2$ spots. A proper domino set never holds the same piece twice. The complete set of size $N$ holds one copy of every possible piece whose marks carry at most $N$ spots. For example, the complete set of size $2$ consists of $6$ pieces.

Write a program that computes the total number of spots on all pieces of the complete set of size $N$.

The first and only line contains the size $N$ of the complete set. ($1 \le N \le 1000$)

Print the total number of spots in the complete set of size $N$.