Исходный код
#include<bits/stdc++.h>

#define INF 1000010000
#define nl '\n'
#define pb push_back
#define ppb pop_back
#define mp make_pair
#define fi first
#define se second
#define pii pair<int,int>
#define pdd pair<double,double>
#define all(c) (c).begin(), (c).end()
#define SORT(c) sort(all(c))
#define rep(i,n) for( int i = 0; i < n; ++i )
#define repi(i,n) for( int i = 1 ; i <= n; ++i )
#define repn(i,n) for( int i = n ; i > 0 ; --i )
#define repf(j,i,n) for( int j = i ; j < n ; ++j )
#define die(s) {std::cout << s << nl;}
#define dier(s) {std::cout << s; return 0;}
#define vi vector<int>
typedef long long ll;

using namespace std;

struct point
{
	int x,y;
	inline point(int _x,int _y){
		x=_x;
		y=_y;
	}
};

inline bool cmp(point a,point b){
	return a.x < b.x || a.x==b.x && a.y < b.y;
}

inline bool cw(point a,point b,point c){
	return ((a.x - b.x) * (c.y - b.y) - (c.x - b.x) * (a.y - b.y)) < 0;
}

inline bool ccw(point a,point b,point c){
	return ((a.x - b.x) * (c.y - b.y) - (c.x - b.x) * (a.y - b.y)) > 0;
}

inline double dist(point a,point b){
	return sqrt(double((a.x-b.x)*(a.x-b.x) + (a.y-b.y)*(a.y-b.y)));
}

int main() {
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
//	cout.precision(1);
	vector<point> v;
	int n;
	cin >> n;
	int x , y;
	rep(i , n){
		cin >> x >> y;
		v.pb(point(x , y));
	}
	sort(all(v) , cmp);
	vector<point> up , down;
	point p1 = v[0] , p2=v.back();
	double ans = 0;
	up.pb(p1);
	down.pb(p1);
	repf(i , 1 , v.size() ){
		if(i == v.size() - 1 || cw(p1 , v[i] , p2)){
			while(up.size() > 1 && !cw(up[up.size() - 2] , up[up.size() - 1] , v[i]))	up.ppb();
		   	up.pb(v[i]);
		}
		if(i == v.size() - 1 || ccw(p1 , v[i] , p2)){
			while(down.size() > 1 && !ccw(down[down.size() - 2] , down[down.size() - 1] , v[i]))	down.ppb();
			down.pb(v[i]);
		}
	}
	v.clear();
	rep(i , up.size()) v.pb(up[i]);
	repn(i , down.size() - 2)	v.pb(down[i]);
	v.pb(v[0]);
	rep(i , v.size() - 1)		ans += dist(v[i] , v[i+1]);
	printf("%.1lf\n", ans);
	return 0;
}