题意：
给出平面上n个点，问能在其中选出6个点，组成两个三角形，使得其互不相交
问有多少种选法
大致思路
考虑枚举一条直线，将所有得点分为左右两部分，其中有两个点在直线上，
以这两个点为顶点，分别统计左边有多少个三角形，右边有多少个三角形即可。
枚举直线时，枚举一个点，然后对其他点极角排序，
枚举另一个点，可快速算出该直线左右两边的点的数量(二分，叉积），组合一下即可。
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