POJ 3348

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

using namespace std;
const int MAXN=10500;
struct point {
	int x,y;
}p[MAXN];

int n;
int ans[MAXN],st[MAXN];
int stop,cnt;

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;
}

bool multi(point a, point b, point c){
	return (a.x-c.x)*(b.y-c.y)-(a.y-c.y)*(b.x-c.x)>0;
}

void slove(){
	stop=cnt=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]])) 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]])) stop--;
		st[stop++]=i;
	}
	for(int i=1;i<stop;i++){
		ans[cnt++]=st[i];
	}
}

int cross(point a,point b,point c){
	return (a.x-c.x)*(b.y-c.y)-(a.y-c.y)*(b.x-c.x);
}

int main(){
	while(scanf("%d",&n)!=EOF){
		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;
	//	cout<<cnt<<endl;
		for(int i=1;i<cnt-2;i++){
			anst+=cross(p[ans[i]],p[ans[i+1]],p[ans[0]]);
		}
		anst=fabs(anst)/2;
	//	cout<<anst<<endl;
		printf("%d\n",int(anst/50));
	}
	return 0;
}