凸包模板
#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cmath>
using namespace std;
const int N=5005;
int n,w,top;
struct dian
{
double x,y;
dian(double X=0,double Y=0)
{
x=X,y=Y;
}
dian operator + (const dian &a) const
{
return dian(x+a.x,y+a.y);
}
dian operator - (const dian &a) const
{
return dian(x-a.x,y-a.y);
}
}a[N],s[N],d;
double cj(dian a,dian b)
{
return a.x*b.y-a.y*b.x;
}
bool cmp(const dian &x,const dian &y)
{
double ar=cj(x-d,y-d);
return ar>0||(ar==0&&x.x<y.x);
}
double dis(dian a,dian b)
{
return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
}
int read()
{
int r=0,f=1;
char p=getchar();
while(p>'9'||p<'0')
{
if(p=='-')
f=-1;
p=getchar();
}
while(p>='0'&&p<='9')
{
r=r*10+p-48;
p=getchar();
}
return r*f;
}
int main()
{
n=read();
d.x=1e9,d.y=1e9;
for(int i=1;i<=n;i++)
{
a[i].x=read(),a[i].y=read();
if(a[i].x<d.x||(a[i].x==d.x&&a[i].y<d.y))
w=i,d=a[i];
}
swap(a[w],a[n]);
n--;
sort(a+1,a+1+n,cmp);
s[++top]=d;s[++top]=a[1];
for(int i=2;i<=n;i++)
{
while(top>1&&cj(s[top]-a[i],s[top-1]-a[i])>=0)
top--;
s[++top]=a[i];
}
double ans=0;
s[top+1]=s[1];
for(int i=1;i<=top;i++)
ans+=dis(s[i],s[i+1]);
printf("%.2lf
",ans);
return 0;
}