想当年……还是邱神给我讲的凸包来着……

#include 
    
     #include 
     
      #include 
      
       #include 
       
        #define eps 0.000000001#define enter putchar('\n')#define space putchar(' ')using namespace std;typedef long long ll;template 
        
         void read(T &x){    char c;    bool op = 0;    while(c = getchar(), c < '0' || c > '9')    if(c == '-') op = 1;    x = c - '0';    while(c = getchar(), c >= '0' && c <= '9')    x = x * 10 + c - '0';    if(op) x = -x;}template 
         
          void write(T x){ if(x < 0) putchar('-'), x = -x; if(x >= 10) write(x / 10); putchar('0' + x % 10);}const int N = 10005, INF = 0x3f3f3f3f;int n, top, ans;struct point { int x, y; point(){} point(int _x, int _y): x(_x), y(_y){} point operator - (point b) const{ return point(x - b.x, y - b.y); } int operator * (point b){ return x * b.y - y * b.x; } int norm(){ return x * x + y * y; }} p[N], stk[N], root;bool operator < (const point &a, const point &b){ int det = (a - root) * (b - root); if(det) return det > 0; return (a - root).norm() < (b - root).norm();}void graham(){ int pos = 0; root.x = root.y = INF; for(int i = 1; i <= n; i++) if(p[i].x < root.x || (p[i].x == root.x && p[i].y < root.y)) root = p[i], pos = i; if(pos != 1) swap(p[pos], p[1]); sort(p + 2, p + n + 1); stk[top = 1] = root; for(int i = 2; i <= n; i++){ while(top >= 2 && (stk[top] - stk[top - 1]) * (p[i] - stk[top - 1]) < 0) top--; stk[++top] = p[i]; }}void getarea(){ stk[top + 1] = stk[1]; for(int i = 1; i <= top; i++) ans += stk[i] * stk[i + 1];}int main(){ read(n); for(int i = 1; i <= n; i++) read(p[i].x), read(p[i].y); graham(); getarea(); write(ans / 2 / 50), enter; return 0;}