Surround the Trees[HDU1392]

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <math.h>
#include <algorithm>
//#include <algorithm>
namespace Geometry {
    class point {
    public:
        double x, y;
        point() {}
        point(const point &p): x(p.x), y(p.y) {}
        point(double a, double b): x(a), y(b) {}
        point operator + (const point & p)const {
            point ret;
            ret.x = x + p.x, ret.y = y + p.y;
            return ret;
        }
        point operator - (const point & p)const {
            point ret;
            ret.x = x - p.x, ret.y = y - p.y;
            return ret;
        }
        //dot product
        double operator * (const point & p)const {
            return x * p.x + y * p.y;
        }
        //cross product
        double operator ^ (const point & p)const {
            return x * p.y - p.x * y;
        }
        bool operator < (const point & p)const {
            if (fabs(x - p.x) < 1e-8) {
                return y < p.y;
            }
            return x < p.x;
        }
        double mold() {
            return sqrt(x * x + y * y);
        }
    };
    double cp(point a, point b, point o) {
        return (a - o) ^ (b - o);
    }
    class line {
    public:
        point A, B;
        line() {}
        line(point a, point b): A(a), B(b) {}
        bool IsLineCrossed(const line &l)const {
            point v1, v2;
            double c1, c2;
            v1 = B - A, v2 = l.A - A;
            c1 = v1 ^ v2;
            v2 = l.B - A;
            c2 = v1 ^ v2;
            if (c1 * c2 >= 0) {
                return false;
            }
            v1 = l.B - l.A, v2 = A - l.A;
            c1 = v1 ^ v2;
            v2 = B - l.A;
            c2 = v1 ^ v2;
            if (c1 * c2 >= 0) {
                return false;
            }
            return true;
        }
    };
    int Graham(point * p, point * s, int n) {
        std::sort(p, p + n);
        int top, m;
        s[0] = p[0];
        s[1] = p[1];
        top = 1;
        for (int i = 2; i < n; i++) { //从前往后扫
            while (top > 0 && cp(p[i], s[top], s[top - 1]) >= 0) {
                top--;
            }
            s[++top] = p[i];
        }
        m = top;
        s[++top] = p[n - 2];
        for (int i = n - 3; i >= 0; i--) { //从后往前扫
            while (top > m && cp(p[i], s[top], s[top - 1]) >= 0) {
                top--;
            }
            s[++top] = p[i];
        }
        return top;
    }
}
//using namespace Geometry;
using namespace Geometry;
point p[200], s[200];
int main() {
#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
#endif
    int n;
    while (scanf("%d", &n) != EOF) {
        if (n == 0) {
            break;
        }
        for (int i = 0; i < n; i++) {
            scanf("%lf%lf", &p[i].x, &p[i].y);
        }
        int cnt = 0;
        for (int i = 1; i < n; i++) //去掉重复的点
            if (fabs(p[i].x - p[cnt].x) > 1e-8 || fabs(p[i].y - p[cnt].y) > 1e-8) {
                p[++cnt] = p[i];
            }
        cnt++;
        if (cnt == 1) {
            printf("0.00\n");
            continue;
        } else if (cnt == 2) {
            printf("%.2lf\n", (p[1] - p[0]).mold());
            continue;
        }
        int top = Graham(p, s, cnt);
        double ans = 0;
        for (int i = 0; i < top - 1; i++) {
            ans += (s[i + 1] - s[i]).mold();
        }
        ans += (s[top - 1] - s[0]).mold();
        printf("%.2lf\n", ans);
    }
    return 0;
}