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凸多边形正则划分问题：对于给定的 $n$ 和 $k$，求一个凸 $nk-2(n-1)$ 边形的 $k$ 角剖分的方案数，对 $10^9+7$ 取模。
每个测试点中包含多组测试数据。
输入的第一行包含两个整数 $n, k$。
输出一行一个整数，表示所有询问的答案之和。
2 3
3 3
3 419对于 $50\%$ 的数据，$n \leq 5\,000$。
对于 $100\%$ 的数据，$k \leq 200, n \leq 1\,100\,000$
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