是Graham的變種。水平掃描，分別構造上下凸殼，然後把兩者接起來。
感覺比Graham好用。
複雜度O(nlog2n) (排序)

#include<cmath>
#include<cstdio>
#include<algorithm>
using namespace std;
const int maxn=1100;
const double eps=1e-9;
int dcmp(double x){
    if(fabs(x)<eps) return 0;
    return x<0?-1:1;
}
struct Point{
    double
 
 x,y;
    Point(double x=0,double y=0):x(x),y(y){}
    bool operator < (const Point &b)const{
        if(x<b.x) return true;
        if(x>b.x) return false;
        return y<b.y;
    }
};
typedef Point Vector;
Vector operator - (Point A,Point B){ return Vector(A.x-B.x,A.y-B.y); }
double
 
 Cross(Vector A,Vector B){ 
    return A.x*B.y-A.y*B.x; 
}
double getDis(Point P1,Point P2){ return sqrt((P1.x-P2.x)*(P1.x-P2.x)+(P1.y-P2.y)*(P1.y-P2.y)); }
int n,tot;
double ans,L;
Point a[maxn],ch[maxn];

int Andrew(){
    sort(a,a+n);
    int len=0;
    for(int i=0;i<=n-1;i++){
        while(len>1
 
&&dcmp(Cross(ch[len]-ch[len-1],a[i]-ch[len-1]))==-1) len--;
        ch[++len]=a[i];
    }
    int k=len;
    for(int i=n-2;i>=0;i--){
        while(len>k&&dcmp(Cross(ch[len]-ch[len-1],a[i]-ch[len-1]))==-1) len--;
        ch[++len]=a[i];
    }
    return len;
}
int main(){
    freopen("poj1113.in","r",stdin);
    freopen("poj1113.out","w",stdout);
    scanf("%d%lf",&n,&L);
    for(int i=0;i<=n-1;i++) scanf("%lf%lf",&a[i].x,&a[i].y);
    int t=Andrew();
    ans=0;
    for(int i=1;i<=t-1;i++) ans+=getDis(ch[i],ch[i+1]);
    printf("%.0lf\n",ans+2*3.14159*L);
    return 0;
}