POJ 1113 Wall （凸包模版题）

好久没写凸包，基本的都忘记了。悲伤。
代码：
#include <iostream>
#include <cstdio>
#include <math.h>
#include <algorithm>
using namespace std;
#define N 1050
#define PI acos(-1.0)
int n,r;
struct point
{
    int x,y;
    point(int _x=0,int _y=0)
    {
        x=_x;
        y=_y;
    }
    point operator-(point b)const
    {
        return point(x-b.x,y-b.y);
    }
}p[N],ch[N];
int cross(point a,point b)
{
    return a.x*b.y-a.y*b.x;
}
bool cmp(point a,point b)
{
    return a.x==b.x?a.y<b.y:a.x<b.x;
}
double dist(point a,point b)
{
    return sqrt((double)(a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
}
int convexhull(point *p,int n,point *ch)
{
    sort(p+1,p+1+n,cmp);
    int m=0;
    for(int i=1;i<=n;++i)
    {
        while(m>1 && cross(ch[m-1]-ch[m-2],p[i]-ch[m-2])<=0)
            m--;
        ch[m++]=p[i];
    }
    int k=m;
    for(int i=n-1;i>=1;--i)
    {
        while(m>k && cross(ch[m-1]-ch[m-2],p[i]-ch[m-2])<=0)
            m--;
        ch[m++]=p[i];
    }
    if(n>1)
        m--;
    return m;
}
int main ()
{
    while(scanf("%d%d",&n,&r)!=EOF)
    {
        for(int i=1;i<=n;++i)
            scanf("%d%d",&p[i].x,&p[i].y);
        int  cnt=convexhull(p,n,ch);
        double res=0.0;
        for(int i=0;i<cnt-1;++i)
            res+=dist(ch[i],ch[i+1]);
        res+=dist(ch[cnt-1],ch[0]);
        res+=2*PI*r;
        printf("%.0lf
",res);
    }

    return 0;
}