The Number of Even Pairs

题目描述

We have $N+M$ balls, each of which has an integer written on it.
It is known that:

写在 $N$ 个球上的数字是偶数。
写在 $M$ 个球上的数字是奇数。

Find the number of ways to choose two of the $N+M$ balls (disregarding order) so that the sum of the numbers written on them is even.
It can be shown that this count does not depend on the actual values written on the balls.

输入格式

Input is given from Standard Input in the following format: $N$ $M$

输出格式

Print the answer.

2 1

1

4 3

9

1 1

0

13 3

81

0 3

3

提示

$0 \leq N,M \leq 100$ $2 \leq N+M$ All values in input are integers.For example, let us assume that the numbers written on the three balls are $1,2,4$.

If we choose the two balls with $1$ and $2$, the sum is odd; If we choose the two balls with $1$ and $4$, the sum is odd; If we choose the two balls with $2$ and $4$, the sum is even.

Thus, the answer is $1$.

标签: AtCoder|abc159A

来源

AtCoder|abc159A