AtCoder Beginner Contest 159
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Problem Statement
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We have $N+M$ balls, each of which has an integer written on it.
It is known that:
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- The numbers written on
$N$of the balls are even. - The numbers written on
$M$of the balls are odd.
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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.
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Constraints
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$0 \leq N,M \leq 100$$2 \leq N+M$- All values in input are integers.
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Input
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Input is given from Standard Input in the following format:
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N M翻译中…
Output
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Print the answer.
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样例
Input #1
2 1
Output #1
1
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 $1$.
Input #2
4 3
Output #2
9
Input #3
1 1
Output #3
0
Input #4
13 3
Output #4
81
Input #5
0 3
Output #5
3
题目信息
题面与样例版权归 AtCoder 及出题人所有；中文翻译由 AI 生成（CI 管线），如有出入请以原文为准。