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Description

有 n + m 个数，其中 n 个奇数， m 个偶数。

从中选择两个数，问有多少种方法使得两个数的和为偶数。

Format

Input

n m

Output

答案

Samples

输入1

2  1

输出1

1

输入2

4 3

输出2

9

输入3

13 3

输出3

81

Limitation

$0 \le n, m \le 100$