凸包维护就可以判断栈顶

#include <iostream>
#include <set>
#include <cmath>
#include <algorithm>

using namespace std;

const double eps=1e-8;

int sgn(double x)
{
    if(fabs(x)<eps)
        return 0;
    if(x<0) return -1;
    else return 1;
}
struct Point{
    double x,y;
    Point(){}
    Point(double _x,double _y)
    {
        x=_x;
        y=_y;
    }
    double operator ^(const struct Point &b)const 
    {
        return x*b.y-y*b.x;
    }
    Point operator -(const struct Point &b)const
    {
        return Point(x-b.x,y-b.y);
    }
};

Point p[111111];
int stack[111111];
set<int> ans;
int n;
void Graham()
{
    stack[0]=0;
    stack[1]=1;
    int top=2;
    for(int i=2;i<n;i++)
    {
        while(top>1&&sgn((p[i]-p[stack[top-2]])^(p[stack[top-1]]-p[stack[top-2]]))>=0)
            top--;
        if(stack[top-1]!=0)//栈顶
            ans.insert(stack[top-1]);
        stack[top++]=i;
    }
}
int main()
{
    scanf("%d",&n);
    for(int i=0;i<n;i++)
    {
        scanf("%lf%lf",&p[i].x,&p[i].y);
    }
    Graham();
    cout<<ans.size()<<endl;
    return 0;
}