POJ 2079

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>

using namespace std;

struct point{
	int x,y;
}p[50050];
int n;

int ans[50050],st[50050],cnt,stop;

bool cmp(point A, point B){
	if(A.y<B.y) return true;
	else if(A.y==B.y){
		if(A.x<B.x) return true;
	}
	return false;
}

int multi(point a,point b,point c){
	point p1; p1.x=a.x-c.x; p1.y=a.y-c.y;
	point p2; p2.x=b.x-c.x; p2.y=b.y-c.y;
	return p1.x*p2.y-p1.y*p2.x;
}

void slove(){
	cnt=stop=0;
	st[stop++]=0; st[stop++]=1;
	for(int i=2;i<n;i++){
		while(stop>1&&multi(p[i],p[st[stop-1]],p[st[stop-2]])>0) stop--;
		st[stop++]=i;
	}
	for(int i=0;i<stop;i++)
	ans[cnt++]=st[i];
	stop=0; st[stop++]=n-1; st[stop++]=n-2;
	for(int i=n-3;i>=0;i--){
		while(stop>1&&multi(p[i],p[st[stop-1]],p[st[stop-2]])>0) stop--;
		st[stop++]=i;
	}
	for(int i=1;i<stop;i++)
	ans[cnt++]=st[i];
}

double Triangle(point a,point b,point c){
	point p1; p1.x=a.x-c.x; p1.y=a.y-c.y;
	point p2; p2.x=b.x-c.x; p2.y=b.y-c.y;
	return fabs((p1.x*p2.y-p1.y*p2.x)*1.0)/2.0;
}

double Area(){
	int q;
	double anst=0;
	for(int i=0;i<cnt;i++){
		q=i+1;
		for(int j=i+1;j<cnt;j++){
		while(Triangle(p[ans[i]],p[ans[j]],p[ans[q]])<Triangle(p[ans[i]],p[ans[j]],p[ans[(q+1)%cnt]]))
		q=(q+1)%cnt;
		anst=max(anst,Triangle(p[ans[i]],p[ans[j]],p[ans[q]]));
		}
	}
	return anst;
}

int main(){
	while(scanf("%d",&n)!=EOF){
		if(n==-1) break;
		for(int i=0;i<n;i++){
			scanf("%d%d",&p[i].x,&p[i].y);
		}
		sort(p,p+n,cmp);
		slove();
		double anst=0;
		anst=max(anst,Area());
		printf("%.2lf\n",anst);
	}
	return 0;
}