Dominoes are gaming pieces used in numerous tile games. Each domino piece contains two marks. Each mark consists of a number of spots (possibly zero). The number of spots depends on the set size. Each mark in a size \(N\) domino set can contain between \(0\) and \(N\) spots, inclusive.

Two tiles are considered identical if their marks have the same number of spots, irregardless of reading order. For example, the tile with \(2\) and \(8\) spot marks is identical to the tile having \(8\) and \(2\) spot marks. A proper domino set contains no duplicate tiles. A complete set of size \(N\) contains all possible tiles with \(N\) or fewer spots and no duplicate tiles. For example, the complete set of size \(2\) contains \(6\) tiles:

$$ [0|0]\quad[0|1]\quad[0|2]\quad[1|1]\quad[1|2]\quad[2|2] $$

Write a program that will determine the total number of spots on all tiles of a complete size \(N\) set.

The first and only line of input contains a single integer, \(N\) (\(1 \le N \le 1000\)), the size of the complete set.

The first and only line of output should contain a single integer, the total number of spots in a complete size \(N\) set.

Second sample description: The size \(3\) set contains the tiles \([0|0]\), \([0|1]\), \([0|2]\), \([0|3]\), \([1|1]\), \([1|2]\), \([1|3]\), \([2|2]\), \([2|3]\) and \([3|3]\).

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출처 COCI 2009/2010 Contest 1