graham扫描法，参考yyb
#include <algorithm>
#include <iostream>
#include <cstdio>
#include <cmath>
using namespace std;
int n, din;
const double eps=1e-7;
struct Point{
double x, y;
Point(double u=0.0, double v=0.0){
x = u; y = v;
}
double crs(Point u){
return x*u.y-y*u.x;
}
Point operator-(const Point &u)const{
return Point(x-u.x, y-u.y);
}
double norm(){
return sqrt(x*x+y*y);
}
}pt[10005], sta[10005];
bool cmp(Point u, Point v){
double tmp=(u-pt[1]).crs(v-pt[1]);
if(fabs(tmp)<eps) return u.norm()<v.norm();
else return tmp>0;
}
int main(){
cin>>n;
for(int i=1; i<=n; i++)
scanf("%lf %lf", &pt[i].x, &pt[i].y);
int ps=1;
for(int i=2; i<=n; i++)
if(pt[i].x<pt[ps].x || (pt[i].x==pt[ps].x && pt[i].y<pt[ps].y))
ps = i;
swap(pt[1], pt[ps]);
sort(pt+2, pt+1+n, cmp);
sta[++din] = pt[1];
for(int i=2; i<=n; i++){
while(din>=2 && (pt[i]-sta[din-1]).crs(sta[din]-sta[din-1])>=-eps)
din--;
sta[++din] = pt[i];
}
sta[din+1] = sta[1];
double ans=0;
for(int i=1; i<=din; i++)
ans += sta[i].crs(sta[i+1]);
ans /= 2;
printf("%d
", (int)ans/50);
return 0;
}