题解：计算凸包……
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cmath>
using namespace std;
struct node{int x,y;}vex[1005],stackf[1005];
bool cmp1(node a,node b){
if(a.y==b.y)return a.x<b.x;
else return a.y<b.y;
}
int cross(node a,node b,node c){return (b.x-a.x)*(c.y-a.y)-(c.x-a.x)*(b.y-a.y);}
double dis(node a,node b){return sqrt((a.x-b.x)*(a.x-b.x)*1.0+(a.y-b.y)*(a.y-b.y));}
bool cmp2(node a,node b){
int m=cross(vex[0],a,b);
if(m==0)return dis(vex[0],a)-dis(vex[0],b)<=0?1:0;
else return m>0?1:0;
}
int main(){
int t;
while(scanf("%d",&t),t!=0){
for(int i=0;i<t;i++)scanf("%d%d",&vex[i].x,&vex[i].y);
if(t==1)printf("%.2f
",0.00);
else if(t==2)printf("%.2f
",dis(vex[0],vex[1]));
else{
sort(vex,vex+t,cmp1);
sort(vex+1,vex+t,cmp2);
memset(stackf,0,sizeof(stackf));
stackf[0]=vex[0]; stackf[1]=vex[1];
int top=1;
for(int i=2;i<t;i++){
while(i>=1&&cross(stackf[top-1],stackf[top],vex[i])<0)top--;
stackf[++top]=vex[i];
}
double s=0;
for(int i=1;i<=top;i++)s+=dis(stackf[i-1],stackf[i]);
s+=dis(stackf[top],vex[0]);
printf("%.2f
",s);
}
}
return 0;
}