#include <cstdio>#include <cstring>#include <cmath>#include <cstdlib>const int MAXN = 109;const double eps = 1e-6;struct point{ double x,y;}p[MAXN],h[MAXN];inline double distance(const point &p1,const point &p2){ return sqrt((p1.x-p2.x)*(p1.x-p2.x)+(p1.y-p2.y)*(p1.y-p2.y));}// 求两点之间的距离inline double multiply(const point &sp,const point &ep,const point &op){ return ((sp.x-op.x)*(ep.y-op.y)-(ep.x-op.x)*(sp.y-op.y));}//判断sp,ep，op是否满足左转int cmp(const void *a,const void *b){//按极角排序 point *p1 = (point *)a; point *p2 = (point *)b; double t = multiply(*p2,*p1,p[0]); if(t>eps) return 1; else if(fabs(t)<=eps) {if(distance(p[0],*p1)>distance(p[0],*p2)) return 1;else return -1;} else return -1;}void anglesort(point p[],int n){//找到最左下方的点 int i,k=0; point temp; for(i=1;i<n;i++) if(p[i].x<p[k].x ||( (p[i].x==p[k].x) && (p[i].y<p[k].y))) k=i; temp=p[0],p[0]=p[k],p[k]=temp; qsort(p+1,n-1,sizeof(point),cmp);}void Graham_scan(point p[],point ch[],int n,int &len){//建立凸包 int i,top=2; anglesort(p,n); if(n<3){ for(i=0,len=n;i<n;i++) ch[i]=p[i]; return; } ch[0]=p[0],ch[1]=p[1],ch[2]=p[2];p[n]=p[0]; for(i=3;i<n;i++){ while(multiply(p[i],ch[top],ch[top-1])>=0) top--; ch[++top]=p[i]; } len=top+1;}int main(){ int i,j,n,len; double d,ans; scanf("%d",&n); for(i=0;i<n;i++) scanf("%lf %lf",&p[i].x,&p[i].y); Graham_scan(p,h,n,len);double sum=0; for(i=1;i<len;i++) { sum+=distance(h[i],h[i-1]); } printf("%.2lf\n",sum); return 0;}
