1 #include <bits/stdc++.h>
  2 
  3 #define debug(x) std::cerr << "[" << __LINE__ << "]: " << #x << " = " << x << "\n"
  4 
  5 using i64 = long long;
  6 
  7 #define LEFT 1
  8 #define RIGHT -1
  9 #define COIN 0
 10 typedef double db;
 11 constexpr db eps = 1e-8;
 12 namespace Geo {
 13     inline db min(db a, db b) { return a < b ? a : b; }
 14     inline db max(db a, db b) { return a > b ? a : b; }
 15     struct Point {
 16         db x, y;
 17         Point(db x = 0, db y = 0) : x(x), y(y) {}
 18         Point operator+(const Point &b) { return Point(x + b.x, y + b.y); }
 19         Point operator-(const Point &b) const { return Point(x - b.x, y - b.y); }
 20         Point operator/(db m) { assert(m != 0); return Point(x / m, y / m); }
 21         db operator*(const Point &b) { return (x * b.x) + (y * b.y); }
 22         db operator^(const Point &b) { return (x * b.y) - (y * b.x); }
 23         friend std::ostream& operator<<(std::ostream& os,const Point& p) {
 24             os << "(" << p.x << "," << p.y << ")"; return os; 
 25         }
 26     };
 27     typedef Point Vector;
 28     struct Line {
 29         Point a, b;
 30         Line(db x1, db y1, db x2, db y2) :a(x1, y1), b(x2, y2) {}
 31     };
 32     inline db cross(Point& o1, Point& a, Point& o2, Point& b) { return (a - o1) ^ (b - o2);}
 33     Point operator*(const Point& a, db m) { return Point(a.x * m, a.y * m); }
 34     Point operator*(db m, const Point& b) { return b * m; }
 35     inline db sqr(db n) { return n * n;}
 36     inline db dist (Point& a, Point& b) { return std::sqrt(sqr(a.x - b.x) + sqr(a.y - b.y));}
 37     inline int dcmp(db x) { return std::abs(x) < eps ? COIN : (x < 0 ? RIGHT : LEFT);}
 38     inline int side(Line& p, Point& q) { return dcmp(cross(p.a, q, p.a, p.b));}
 39     bool onSegment(const Point& p1, const Point& p2, const Point& q) { return dcmp((p1 - q) ^ (p2 - q)) == 0 && dcmp((p1 - q) * (p2 - q)) <= 0;}
 40     bool onSegment(const Line& l,const Point& q) { return onSegment(l.a, l.b, q); }
 41     struct Polygon {
 42         std::vector<Point> poly;
 43         Polygon() {}
 44         void add_point(Point& p) { poly.emplace_back(p); }
 45         void add_point(db x, db y) { poly.emplace_back(x, y); }
 46         friend std::ostream& operator<<(std::ostream& os, const Polygon& obj) {
 47             for (int i = 0, sz = (int)obj.poly.size(); i < sz; i++) {
 48                 os << obj.poly[i] << " ";
 49             }
 50             return os;
 51         }
 52         bool inside(const Point& p) {
 53             bool flag = false;
 54             for (int i = 0, sz = (int)poly.size(), j = sz - 1; i < sz; j = i++) {
 55                 Point p1 = poly[i];
 56                 Point p2 = poly[j];
 57                 if (onSegment(p1, p2, p)) return true;
 58                 if ((dcmp(p1.y - p.y) > 0) != (dcmp(p2.y - p.y) > 0) \
 59                     && dcmp(p.x - (p.y - p1.y) * (p1.x - p2.x) / (p1.y - p2.y) - p1.x) < 0 )
 60                     {
 61                         flag = !flag;
 62                     }
 63             }
 64             return flag;
 65         }
 66         db Circumstance() {
 67             db ans = 0;
 68             for (int i = 0, sz = (int)poly.size(), j = sz - 1; i < sz; j = i++) {
 69                 ans += dist(poly[i], poly[j]);
 70             }
 71             return ans;
 72         }
 73         db Area(){
 74             db ans = 0;
 75             Point assist(0, 0);
 76             for (int i = 0, sz = (int)poly.size(), j = sz - 1; i < sz; j = i++) {
 77                 ans += cross(assist, poly[i], assist, poly[j]);
 78             }
 79             return std::abs(ans) * 0.5;
 80         }
 81     };
 82    
 83     bool intersect(Line& l1, Line& l2) {
 84         if (max(l1.a.x, l1.b.x) < min(l2.a.x, l2.b.x)) return false;//排斥实验 
 85         if (max(l1.a.y, l1.b.y) < min(l2.a.y, l2.b.y)) return false;
 86         if (max(l2.a.x, l2.b.x) < min(l1.a.x, l1.b.x)) return false;
 87         if (max(l2.a.y, l2.b.y) < min(l1.a.y, l1.b.y)) return false;
 88         if (cross(l1.a, l2.a, l1.a, l1.b) * cross(l1.a, l1.b, l1.a, l2.b) < 0) return false;//跨立实验
 89         if (cross(l2.a, l1.a, l2.a, l2.b) * cross(l2.a, l2.b, l2.a, l1.b) < 0) return false;
 90         return true;
 91     }
 92     Point interpoint(Point& a1, Point& a2, Point& b1, Point& b2) {
 93         Vector v1 = Point(a2.x - a1.x, a2.y - a1.y);
 94         Vector v2 = Point(b2.x - b1.x, b2.y - b1.y);
 95         Vector u = a1 - b1;
 96         db t = (v2 ^ u) / (v1 ^ v2);
 97         return Point(a1.x + v1.x * t, a1.y + v1.y * t);
 98     }
 99     Point interpoint(Line& l1, Line& l2) {
100         return interpoint(l1.a, l1.b, l2.a, l2.b);
101     }
102     
103 };
104 using namespace Geo;
105 inline void swap(db& a, db& b) { db t = a; a = b; b = t;}
106 const int N = 2e5 + 5;
107 Point p[N];
108 int main() {
109    
110     int n;
111     scanf("%d", &n);
112     
113     for (int i = 1; i <= n; i++) {
114   
115         scanf("%lf %lf", &p[i].x, &p[i].y);
116 
117     }
118     std::sort(p + 1, p + n + 1, [&](Point& a, Point& b){
119         
120         if (std::abs(a.y - b.y) < eps) return a.x < b.x;
121         return a.y < b.y;
122     });
123     std::sort(p + 2, p + n + 1, [&](Point& pa, Point& pb) {
124         double x = cross(p[1], pa, p[1], pb);
125         if (x > eps) return true;//若大于0，就说明 p_pa 在 p_pb的左边 (根据右手螺旋定则)
126         if (std::abs(x) < eps && dist(p[0], pa) < dist(p[0], pb)) return true;
127         return false;
128     });
129     
130     Point* st = new Point [200005];
131     
132     int cnt = 1;
133     st[cnt] = p[1];
134 
135     for (int i = 2; i <= n; i++) {
136         while (cnt > 1 && cross(st[cnt - 1], st[cnt], st[cnt], p[i]) <= 0) {
137             --cnt;
138         }
139         ++cnt;
140         st[cnt] = p[i];
141     }
142     st[cnt + 1] = p[1];
143     double ans = 0;
144     for (int i = 1; i <= cnt; i++) {
145         ans += dist(st[i], st[i + 1]);
146     }
147     printf("%.2f\n", ans);
148     delete []st;
149     return 0;
150 }