Always WA#3，不同方式排序不同输出

# include <bits/stdc++.h>
//#define int long long
using namespace std;
typedef long long ll;
typedef pair <int, int> pii;

int n, lst;

double lst1[100005], lst2[100005], sum, l = 1e6;

pair <double, double> a[100005];

stack <int> st;

bool cmp (const pair <double, double>& a, const pair <double, double>& b) {

	return a.second != b.second ? a.second < b.second : a.second <= l ? a.first < b.first: a.first > b.first;

}

double dis (const double x, const double y) {

	return sqrt (x * x + y * y);

}

int main () {

	ios::sync_with_stdio (0);
	cin.tie (0);
	cout.tie (0);

	cin >> n;

	for (int i = 1; i <= n; ++ i)
		cin >> a[i].first >> a[i].second, l = min (l, a[i].second);
	sort (a + 1, a + 1 + n);
//	sort (a + 1, a + 1 + n, cmp);

	lst1[1] = lst2[1] = 0, st.push (1);

	for (int i = 2; i <= n; ++ i) {
//		(a[i].first - a[st.top ()].first) / (a[i].second - a[st.top ()].second) <= lst1 / lst2
//		cerr << i << ':' << a[i].first << ',' << a[i].second << ':' << st.size () << '\n';
//		cerr << (a[i].first - a[st.top ()].first) * lst2 << ',' << lst1 << '*' << (a[i].second - a[st.top ()].second) << '\n';
//		cerr << (a[i].first - a[1].first) * lst2 << ',' << lst1 << '*' << (a[i].second - a[1].second) << '\n';
		while ((a[i].first - a[st.top ()].first) * lst2[st.top ()] >= lst1[st.top ()] * (a[i].second - a[st.top ()].second))
			st.pop ();
//		cerr << i << ':' << a[i].first << ',' << a[i].second << ':' << st.size () << ':' << st.top () << '\n';
		lst1[i] = a[i].first - a[st.top ()].first, lst2[i] = (a[i].second - a[st.top ()].second);

		st.push (i);

	}

	lst = st.top (), st.pop ();

	while (! st.empty ()) //cerr << st.top () << "->" << lst << ':' << a[st.top ()].first << ',' << a[st.top ()].second << "->" << a[lst].first << ',' << a[lst].second << '\n',
		sum += dis (a[lst].first - a[st.top ()].first, a[lst].second - a[st.top ()].second), lst = st.top (), st.pop ();
//	cerr << sum << '\n';
	/*--------------------------------------------*/
	lst1[n] = lst2[n] = 0, st.push (n);

	for (int i = n - 1; i; -- i) {
		//(a[i].first - a[st.top ()].first) / (a[i].second - a[st.top ()].second) <= lst1 / lst2
		while ((a[i].first - a[st.top ()].first) * lst2[st.top ()] >= lst1[st.top ()] * (a[i].second - a[st.top ()].second))
			st.pop ();

		lst1[i] = a[i].first - a[st.top ()].first, lst2[i] = (a[i].second - a[st.top ()].second);

		st.push (i);

	}

	lst = st.top (), st.pop ();

	while (! st.empty ())
		sum += dis (a[lst].first - a[st.top ()].first, a[lst].second - a[st.top ()].second), lst = st.top (), st.pop ();

	cout << fixed << setprecision (2) << sum;

	return 0;
}