Score : $300$ points
There is a grid with infinitely many rows and columns. In this grid, there is a rectangular region with consecutive $N$ rows and $M$ columns, and a card is placed in each square in this region. The front and back sides of these cards can be distinguished, and initially every card faces up.
We will perform the following operation once for each square contains a card:
It can be proved that, whether each card faces up or down after all the operations does not depend on the order the operations are performed. Find the number of cards that face down after all the operations.
Input is given from Standard Input in the following format:
Print the number of cards that face down after all the operations.
We will flip every card in any of the four operations. Thus, after all the operations, all cards face up.
After all the operations, all cards except at both ends face down.