1 #include <stdio.h> 2 #include <string.h> 3 #include <iostream> 4 #include <math.h> 5 #include <algorithm> 6  7 using namespace std ; 8  9 struct point10 {11     int x,y ;12 }p[110],p1[110];13 int n ;14 15 double dis(point a,point b)16 {17     return sqrt(double((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y))) ;18 }19 double cross(point a,point b,point c)20 {21     return (a.x-c.x)*(b.y-c.y) - (a.y-c.y)*(b.x-c.x) ;22 }23 bool cmp(point a,point b)24 {25     if(cross(a,b,p[0]) > 0 || cross(a,b,p[0]) == 0 && dis(a,p[0]) < dis(b,p[0]))26         return true ;27     return false ;28 }29 void Graham()30 {31     int cnt;32     sort(p+1,p+n,cmp) ;33     p[n] = p[0] ;34     p1[0] = p[0] ;35     p1[1] = p[1] ;36     cnt = 1 ;37     for(int i = 2 ; i <= n ; i++)38     {39         while(cnt >= 1 && cross(p1[cnt-1],p1[cnt],p[i]) <= 0) cnt -- ;40         p1[ ++cnt] = p[i] ;41     }42     double sum = 0.0 ;43     for(int i = 0 ; i < cnt ; i++)44         {45             sum += dis(p1[i],p1[i+1]) ;46             //printf("*%.2lf*\n",sum) ;47         }48         printf("%.2lf\n",sum) ;49 }50 int main()51 {52     while(~scanf("%d",&n) && n)53     {54         int pos = 0 ;55         for(int i = 0 ; i < n ; i++)56         {57             scanf("%d %d",&p[i].x,&p[i].y) ;58             if(p[pos].y > p[i].y || (p[pos].y == p[i].y && p[i].x < p[pos].x))59                 pos = i ;60         }61         if(n == 1) {puts("0.00");continue ;}62         if(n == 2)63         {64             printf("%.2lf\n",dis(p[1],p[0])) ;65             continue ;66         }67         swap(p[pos],p[0]) ;68         //printf("%d %d\n",p[0].x,p[0].y) ;69         Graham() ;70     }71     return 0 ;72 }