学步园

#include<bits/stdc++.h>
using namespace std;
struct P{
	double x,y;
}p[2001],s[2001];
int n,top;
inline double dis(P a,P b){
	return (a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y);
}
inline P operator-(P a,P b){
	return (P){a.x-b.x,a.y-b.y};
}
inline double operator*(P a,P b){
	return a.x*b.y-b.x*a.y; 
} 
inline bool operator<(P a,P b){
	double t=(a-p[1])*(b-p[1]);
	if(t==0)return dis(a,p[1])<dis(b,p[1]);
	return t<0;
}
inline void graham(){
	int k=1;
	for(int i=2;i<=n;i++)
		if(p[k].y>p[i].y||(p[k].y==p[i].y&&p[k].x>p[i].x))
			k=i;
	swap(p[1],p[k]);
	sort(p+2,p+n+1);
	s[++top]=p[1];s[++top]=p[2];
	for(int i=3;i<=n;i++){
		while(top>1&&(p[i]-s[top-1])*(s[top]-s[top-1])<=0)
			top--;
		s[++top]=p[i];
	}
}
inline double RC(){
	s[top+1]=p[1];
	double ans=0;
	int a,b;
	for(int x=1;x<=top;x++){
		a=x%top+1;b=(x+2)%top+1;
		for(int y=x+2;y<=top;y++){
			while(a%top+1!=y&&(s[y]-s[x])*(s[a+1]-s[x])>(s[y]-s[x])*(s[a]-s[x]))
                a=a%top+1;
			while(b%top+1!=x&&(s[b+1]-s[x])*(s[y]-s[x])>(s[b]-s[x])*(s[y]-s[x]))
                b=b%top+1;
            ans=max((s[y]-s[x])*(s[a]-s[x])+(s[b]-s[x])*(s[y]-s[x]),ans);
        }
	}
	return ans;
}
int main(){
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
        scanf("%lf%lf",&p[i].x,&p[i].y);
    graham();
    printf("%.3lf",RC()/2);
    return 0;
}