由於xy範圍比較小，所以可以列舉斜率進行計算。

#include<bits/stdc++.h>
using namespace std;

const int MAXN=2020;
const int inf=0x3f3f3f3f;
const double eps=1e-6;
long long x[MAXN],y[MAXN];
double slope[MAXN*MAXN];

int main()
{
	long long n,i,j,last,now,tmp,k,rep;
	double res,dx,dy;
	while(~scanf("%lld",&n))
	{
		for(i=1;i<=n;i++)
			scanf("%lld%lld",&x[i],&y[i]);
		rep=0;
		for(i=1;i<=n;i++)
		{
			k=0;
			for(j=i+1;j<=n;j++)
			{
				dx=x[j]-x[i];
				dy=y[j]-y[i];
				if(dx==0)
					res=inf;
				else
					res=dy/dx;
				slope[k++]=res;
			}
			sort(slope,slope+k);
			last=0;
			now=1;
			for(;now<k;now++)
			{
				if(abs(slope[now]-slope[last])>eps)
				{
					tmp=now-last;
					rep+=tmp*(tmp-1)/2;
					last=now;
				}
			}
			tmp=now-last;
			rep+=tmp*(tmp-1)/2;
		}
		printf("%lld\n",n*(n-1)*(n-2)/6-rep);
	}
}