C++ :

#include<cstdio>
#include<algorithm>
#include<iostream>
#include<cmath>
using namespace std;
int n,ss;
struct point
{
	double x,y;
}p[10005],s[10005];
bool cmp(point p1,point p2)
{
	if(p1.y==p2.y)
	return p1.x<p2.x;
	return p1.y<p2.y;
}
int mult(point p0,point p1,point p2)
{
	return (p1.x-p0.x)*(p2.y-p0.y)>=(p2.x-p0.x)*(p1.y-p0.y);
}
int graham()
{
	int i,j,k,top=1,mint;
	s[0]=p[0];s[1]=p[1];
	for(i=2;i<n;i++)
	{
		while(top&&mult(p[i],s[top],s[top-1]))
		top--;
		s[++top]=p[i];
	}
	mint=top;s[++top]=p[n-2];
	for(i=n-3;i>=0;i--)
	{
		while(top!=mint&&mult(p[i],s[top],s[top-1]))
		top--;
		s[++top]=p[i];
	}
	return top;
}
double f(point p1,point p2)
{
	return sqrt((p1.x-p2.x)*(p1.x-p2.x)+(p1.y-p2.y)*(p1.y-p2.y)); 
}
void solve()
{
	int i;
	double ans=0;
	ss=graham();
	for(i=0;i<ss-1;i++)
	{
		ans+=f(s[i],s[i+1]);
	}
	ans+=f(s[0],s[ss-1]);
	printf("%.2lf\n",ans);
}
int main()
{
	int i;
	scanf("%d",&n);
	for(i=0;i<n;i++)
	scanf("%lf %lf",&p[i].x,&p[i].y);
	if(n==0||n==1)
	printf("0.00\n");
	else if(n==2)
	printf("%.2lf\n",f(p[0],p[1]));
	else
	{
		sort(p,p+n,cmp);
		solve();	
	}
	return 0;
}