1 #include <cstdio>
 2 #include <algorithm>
 3 #include <cstring>
 4 #include <cmath>
 5 #include <stack>
 6 #include <queue>
 7 #include <iostream>
 8 #define mod 9999973
 9 #define N 100005
10 using namespace std;
11 int n , top;
12 double ans;
13 struct point
14 {
15     int x , y;
16     bool operator < (const point & a) const
17     {
18         if(x != a.x) return x < a.x;
19         return y < a.y;
20     }
21 }e[N] , sta[N];
22 double get(point p1 , point p2 , point p3)
23 {
24     return (p1.x - p3.x) * (p2.y - p3.y) - (p2.x - p3.x) * (p1.y - p3.y);
25 }
26 double dist(point p1 , point p2)
27 {
28     return sqrt((p1.x - p2.x) * (p1.x - p2.x) + (p1.y - p2.y) * (p1.y - p2.y));
29 }
30 void work()
31 {
32     scanf("%d", &n);
33     for(int i = 1 ; i <= n ; ++i) scanf("%d%d", &e[i].x, &e[i].y);
34     sort(e + 1 , e + 1 + n);
35     sta[++top] = e[1] , sta[++top] = e[2];
36     for(int i = 3 ; i <= n ; ++i)
37     {
38         while(top >= 2 && get(sta[top] , e[i] , sta[top - 1]) <= 0) --top;
39         sta[++top] = e[i];
40     }
41     int len = top;
42     sta[++top] = e[n - 1];
43     for(int i = n - 2 ; i >= 1 ; --i)
44     {
45         while(len != top && get(sta[top] , e[i] , sta[top - 1]) <= 0) --top;
46         sta[++top] = e[i];
47     }
48     ans += dist(sta[1] , sta[top]);
49     for(int i = 1 ; i < top ; ++i) ans += dist(sta[i] , sta[i + 1]);
50     printf("%.1f", ans);
51 }
52 int main()
53 {
54     work();
55     return 0;
56 }