Consider N-dominoes. In such a domino, each domino consists of two halves, each of which is drawn from 0 to N dots. A complete set of bones of such a domino contains all possible bones, each — once. For example, for N=2, the set will include the following tiles: (0.0), (0.1), (0.2), (1.1), (1.2) and (2.2)
Write a program that, given N, will determine how many dots are shown on all tiles of a complete set of N-dominoes.
Input
A natural number N is entered (1<=N<=30).
Imprint
The program should print one number - the total number of dots on all tiles of a complete set of N-dominoes.
Examples