CF1725A.Accumulation of Dominoes

Pak Chanek has a grid that has $N$ rows and $M$ columns. Each row is numbered from $1$ to $N$ from top to bottom. Each column is numbered from $1$ to $M$ from left to right.

Each tile in the grid contains a number. The numbers are arranged as follows:

• Row $1$ contains integers from $1$ to $M$ from left to right.
• Row $2$ contains integers from $M+1$ to $2 \times M$ from left to right.
• Row $3$ contains integers from $2 \times M+1$ to $3 \times M$ from left to right.
• And so on until row $N$.

A domino is defined as two different tiles in the grid that touch by their sides. A domino is said to be tight if and only if the two numbers in the domino have a difference of exactly $1$. Count the number of distinct tight dominoes in the grid.

Two dominoes are said to be distinct if and only if there exists at least one tile that is in one domino, but not in the other.

帕克·查内克有一个 $N$ 行 $M$ 列的网格。各行从上到下编号为 $1$ 至 $N$，各列从左到右编号为 $1$ 至 $M$。

网格中每个格子包含一个数字，这些数字按如下方式排列：

• 第 $1$ 行从左到右依次为整数 $1$ 至 $M$；
• 第 $2$ 行从左到右依次为整数 $M+1$ 至 $2 \times M$；
• 第 $3$ 行从左到右依次为整数 $2 \times M+1$ 至 $3 \times M$；
• 依此类推，直至第 $N$ 行。

一个“多米诺骨牌”（domino）定义为网格中两个相邻（即共享一条边）的不同格子。当且仅当这两个格子中的数字之差的绝对值恰好为 $1$ 时，该多米诺骨牌被称为“紧凑的”（tight）。请计算网格中不同的紧凑多米诺骨牌的总数。

当且仅当存在至少一个格子属于其中一个多米诺骨牌但不属于另一个时，称这两个多米诺骨牌是不同的。

The only line contains two integers $N$ and $M$ ($1 \leq N, M \leq 10^9$) — the number of rows and columns in the grid.

唯一一行包含两个整数 $N$ 和 $M$（$1 \leq N, M \leq 10^9$）—— 分别表示网格的行数和列数。

An integer representing the number of distinct tight dominoes in the grid.

表示网格中不同紧密多米诺骨牌数量的整数。

The picture below is the grid that Pak Chanek has in the first example.

The picture below is an example of a tight domino in the grid.

下图是 Pak Chanek 在第一个样例中所拥有的网格。

下图是该网格中一个紧凑多米诺骨牌（tight domino）的示例。