31372번 - Microcircuits 다국어

문제

You probably know how microcircuits look like. First of all it is important to pay special attention to connections. Contacts on the circuit are connected by lines. If two lines do not intersect then there is no connection between respective contacts.

You are holding a circular circuit with $n$ contacts around the borderline. You have to calculate the number of possibilities to put exactly $k$ non-intersecting lines each connecting two contacts.

입력

Single line contains two integers $n$ and $k$ (1ドル \le k \le n \le 40$).

출력

Output the required number.

제한

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