Tiling with Hexagons

给定一个由正六边形拼成的不规则六边形的边长$a,b,c$，求该新六边形由多少个正六边形构成的。 例如下图中$a=2,b=3,c=4$，它由$18$个正六边形组成。 

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 $ a $ , $ b $ , $ c $ , $ a $ , $ b $ and $ c $ adjacent tiles, correspondingly. To better visualize the situation, look at the picture showing a similar hexagon for $ a=2 $ , $ b=3 $ and $ c=4 $ . According to the legend, as the King of Berland obtained the values $ a $ , $ b $ and $ c $ , he almost immediately calculated the total number of tiles on the hall floor. Can you do the same?

The first line contains three integers: $ a $ , $ b $ and $ c $ ( $ 2<=a,b,c<=1000 $ ).

Print a single number — the total number of tiles on the hall floor.

2 3 4

18