参考例题：http://poj.org/problem?id=3348
备注：可以处理重点，坐标值可以是整数和小数
#include<iostream>
#include<cstdio>
#include<cmath>
#include<algorithm>
#define LL long long
using namespace std;
const int N = 100000;
const double EPS = 1e-8;
int n,cnt;
struct Point{
double x,y;
inline Point() {}
inline Point(double a, double b):x(a),y(b) {}
inline Point operator + (const Point &A) {return Point(x+A.x, y+A.y);}
inline Point operator - (const Point &A) {return Point(x-A.x, y-A.y);}
inline Point operator / (double A) {return Point(x/A, y/A);}
inline Point operator * (double A) {return Point(x*A, y*A);}
inline double operator * (const Point &A) {return x*A.x + y*A.y;}
inline double operator ^ (const Point &A) {return x*A.y - A.x*y;}
inline bool operator < (const Point &B) const {return x < B.x - EPS || (fabs(x-B.x) < EPS && y < B.y - EPS);}
inline bool operator == (const Point &B) const {return fabs(x - B.x) + fabs(y - B.y) < EPS;}
}p[N],cvx[N];
inline int read() {
char c=getchar(); int f=1,ret=0;
while (c<'0'||c>'9') {if(c=='-')f=-1;c=getchar();}
while (c<='9'&&c>='0') {ret=ret*10+c-'0';c=getchar();}
return ret * f;
}
inline void ConvexHull(int tot, Point *a, int &sz, Point *ret) {
if (!tot) {sz = 0; return;}
sort(a+1, a+1+tot); ret[sz=1] = a[1];
tot = unique(a+1, a+1+tot) - a - 1;
for (int i=2;i<=tot;i++) {
while (sz > 1 && ((ret[sz] - ret[sz-1]) ^ (a[i] - ret[sz])) < EPS) sz--;
ret[++sz] = a[i];
}
for (int i=tot-1,mn=sz;i>0;i--) {
while (sz > mn && ((ret[sz] - ret[sz-1]) ^ (a[i] - ret[sz])) < EPS) sz--;
ret[++sz] = a[i];
}
sz -= sz > 2;
}
int main() {
n = read();
for (int i=1;i<=n;i++) p[i].x = read(), p[i].y = read();
ConvexHull(n, p, cnt, cvx);
double vout = 0;
for (int i=2;i<cnt;i++)
vout += (cvx[i] - cvx[1]) ^ (cvx[i+1] - cvx[1]) / 2;
printf("%d\n",(int)(vout/50));
return 0;
}
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