1100.Tiling with Hexagons
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1100.Tiling with Hexagons

时间限制:1000MS  内存限制:65535KB
题目描述

Several ages ago Berland was a kingdom. The King of Berland adored math. That's why, when he first visited one of his many palaces, he first of all paid attention to the floor in one hall. The floor was tiled with hexagonal tiles.

The hall also turned out hexagonal in its shape. The King walked along the perimeter of the hall and concluded that each of the six sides has abcab and c adjacent tiles, correspondingly.

To better visualize the situation, look at the picture showing a similar hexagon for a=2b=3 and c=4.


According to the legend, as the King of Berland obtained the values ab and c, he almost immediately calculated the total number of tiles on the hall floor. Can you do the same?

输入
The first line contains three integers: ab and c (2≤a,b,c≤1000).
输出
Print a single number — the total number of tiles on the hall floor.
示例
input
2 3 4
output
18
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